Proceedings of the Washington Academy of Sciences
Vol. VIII, pp. 197‑403. February 13, 1907.
ASPECTS OF KINETIC EVOLUTION
By O. F. COOK
2. INTRASPECIFIC DIFFERENCES AS MATERIALS OF EVOLUTION.
The time has gone by when it was supposed that new knowledge could be gained by the analysis and rearrangement of old data and deductions. Nevertheless, it remains true that every advance in science requires, sooner or later, a new and consistent arrangement of the materials of investigation, and of the language to be used in describing them. Words are not things, but they often control the predisposition of the mind and thus obscure or illuminate the field of mental vision. 
Science deals primarily with facts, and only incidentally with inferences or theories, though the latter are of immense use in helping to ascertain facts and test their causal relations. Useful theories arrange facts in what appear to be connected sequences, and enable us to project ourselves into the realm of the unknown without hopelessly losing our way in the maze of unrelated data which we are otherwise likely to encounter. We follow the theory until we encounter facts which prove or disprove it, or until a more direct or more coherent theory has been suggested.
Theories are like legislative enactments; the surest way to be rid of a bad one is to enforce it. A false theory, if studied with sufficient care will correct itself, because the places will be found where it is inapplicable. Moreover, the theories and laws which are the most difficult to repeal are those which contain a large measure of truth and justice, and which have been long in force, so that many vested interests have grown up around them. They take possession, as it were, of the field of investigation, divide it up and place on guard a multitude of technical terms and distinctions which defend the approaches of the citadel of error by a battery of words, which go far to keep a new idea unintelligible.
The prevalent doctrine that evolution is caused or actuated by natural selection is such a theory, containing a large and important truth, and at first immensely fertile in scientific results and practical applications, but essentially erroneous, and in some fundamental respects dangerous to agriculture and to man himself.
The basal axioms, the things taken for granted in the selection theory are (1) that species are normally stationary and constant in their characters and (2) that their evolutionary progress is caused by the environment, but neither of these assumptions proves to accord with the facts. It has not been shown that either environment itself or the selection which it exerts are true, efficient causes of evolution. Neither has evidence been found to prove that a species has ever remained stationary in all its characters, or that the component individuals tend to become "exactly alike," even under the most uniform conditions. 
Nature abounds in striking evidence of the alternative kinetic view that species are normally in motion, and that the individual organisms of which they are composed have a normal and necessary intraspecific diversity, quite independent of environmental influences. Moreover, there is reason to believe, from the prevalence of sexual and other diversities inside the specific lines, and from the degeneration which follows attempts at maintaining a stable and uniform type, that diversity among individuals of a species is not only universal and normal, but necessary and advantageous. The prevalent doctrine that evolution is caused or actuated by natural selection has been characterized as a static theory because species are thought of as normally at rest, that is, as stationary or constant in characters and tending to be uniform as far as external conditions will permit. The causes of variation and of evolution were sought in the environment and not in the species itself. The problem was to show how the external causes produce the internal effects, but the task was hopeless from the beginning, for the variations which the environment causes are not those through which evolution goes forward.
It is apparent, therefore, that the abandonment of the static point of view, and the placing of a new interpretation upon a large class of familiar facts calls for a new plan for the study and discussion of the phenomena familiarly called variations, in the older and looser sense of the term, meaning all the differences to be found among the individuals of a species. Differences not caused by environmental influences were, of course, quite unconsidered in static theories and classifications. There was not even a scientific term for this universal phenomenon of intraspecific diversity.
A complete treatment of the subject would involve the rearrangement of a large part of the data which have figured in the evolutionary literature of the last half‑century. The scope of the present statement permits only a brief and imperfect outline. It is not possible even to adequately describe and illustrate the details of the facts of original observation to which reference is made. Particular instances are not given, therefore, with any idea that they are adequate to demonstrate the truth of the interpretation  which has been put upon them. They serve only as samples of groups of facts to which the interpretation is applicable, the primary object being, not to demonstrate conclusions by formal arguments, but to indicate a standpoint, the correctness of which may be judged by other observers from the facts encountered in their own fields of investigation.
To learn the nature and causes of evolution it has not been sufficient to explore and explain the barriers between the species. It is necessary to go inside the species and to ascertain, if possible, which of the many differences between the component individuals represent forward steps in organic development, and which mere lateral diversions or displacements.
DARWIN'S DISCOVERY OF VARIATION.
Much has been written to show that Darwin did not discover evolution, as popularly supposed, since the idea may be traced back to the Greek philosophers or to the Hindus, and had been entertained in modern times by Lamarck and several others of Darwin's predecessors. And yet, the popular impression, though perhaps inexact as to technical terms, is more just than that of many scientific critics. Darwin was able to secure general interest and confidence in an idea previously indefinite, intangible and practically useless. If Darwin did not discover evolution or even invent entirely new arguments in its favor, he performed a more valuable and unique service in establishing the fundamental fact of variation, without which all evolutionary ideas would have remained empty and sterile speculations, as they had remained during the two thousand years preceding.
Darwin discovered what is still more important to the scientific world than the abstract idea or theory of evolution, namely the means of evolution, which is variation. Darwin was the first to adequately appreciate the fact that species do not consist of individuals identical in form or structure, but of those which are diverse, each different from the others in a greater or lesser degree. Upon the fact of variation Darwin also based his theory of evolution by natural selection and other environmental causes, a theory which has had great popularity in the general scientific world, because it afforded the most concrete suggestion  regarding the nature of the causes of evolution. It is desired therefore, to distinguish clearly at this point between the facts of variation first adequately recognized by Darwin and the theory of environmental causes of evolution often called Darwinism. Naturalists do not all believe in environmentally caused evolution, but nearly all are now agreed in thinking of species, not as single morphological points, but as large groups of similar individuals.
Since the time of Darwin it has been believed that evolution has been accomplished by means of variations, but there is still the widest divergence of scientific opinion regarding the kinds of variations which cause or contribute to developmental changes. Some theories depend upon one or another of the different kinds of variations and ignore the others, and some hold that all variations are caused by the environment and that evolution itself is merely a summary of environmental influences.
Many writers have approached the subject from the standpoint of formal definitions and narrowly technical distinctions, but the practical divergences between the different views become most apparent from the types of variation—the kinds of intraspecific differences—upon which they depend as showing the nature of evolutionary motion. To correctly fix upon the kind or kinds of variations which contribute to evolution, is the first step of progress toward knowledge of the true evolutionary factors, and brings us by the most direct route to the determination of the primary question, whether the true, efficient causes of evolution lie in the environment or in the organisms themselves. Are the variations which are induced by the environment those by which evolutionary progress is accomplished?
In Darwin's original suggestion environment was held to bring about evolution, first by inducing variations and then by selecting those which proved to be advantageous. The environment was considered as at once the cause of variations and of evolution. This view is still generally accepted as the teaching of science regarding organic evolution, although many modifications and collateral suggestions have appeared necessary to Darwin himself and to many of his successors. Some have approached the Lamarckian idea of direct adaptation, in ascribing  much to the moulding influence of the environment, and in requiring correspondingly little of selection. Other writers have gone to the opposite extreme, making little of environmental factors and much of natural selection of fortuitous individual variations. The latter tendency has been dominant since Weismann showed that "acquired characters," the results of direct environmental influences, are seldom or never inherited.
In the original Darwinism and its various amended forms there seems usually to have been included the tacit assumption of a constant of variability. It is taken for granted that a certain amount of variation shall be manifested by each species, so that selection by paring off the species on one side can cause it to grow out on the other, and thus compel a gradual change of characters. Without selection the average is thought to remain stationary, and if selection be withdrawn the progress already made may be lost by retrogression. Selection, in this view, is the true actuating cause or principle of evolution.
Mivart, and recently many others, have considered that both the environmental variations and the minute and fluctuating individual differences were alike in adequate to accomplish evolution through selection, and have advocated a return toward the older doctrine of special creation. They hold still to the evolutionary idea that species arise one from another, but suppose that the new types originate suddenly by "extraordinary births," or by abrupt mutative variations, that is, by individuals which depart widely from the type of the older species. The occurrence of many such abrupt variations is a definitely established fact. Among plants they often come true to seed, and among animals they are often prepotent when bred with other members of their own variety or local species. Nevertheless, it does not appear that this is the method by which species originate in nature. The prepotency of new variations indicates the probability that old species are tranformed by this means rather than that new species are abruptly originated.
|1Cook, O. F., 1902. Evolutionary Inferences from the Diplopoda. Proc. Entomological Society of Washington, 5:14.|
Darwin appreciated better than many of his successors in the field of evolutionary literature the fact that variations are of many kinds, of very different evolutionary significance, and due to many different causes. As an evolutionary pioneer it was  a sufficient service to have shown that enough variation exists to make evolution feasible or even plausible. The scholastically educated public, which often appreciates arguments much better than facts, was obliged to approach evolution through Darwin's deductions rather than through his perceptions. Evolution was accepted or rejected on the merits of natural selection, though the two ideas have no necessary connection. Natural selection and evolution are both facts, but in proving that the one is the adequate practical cause of the other it would be necessary to show that the variations through which evolution goes forward are caused by natural selection. No such causation has been demonstrated. Natural selection does not furnish the variations nor explain why variations are accumulated and carried forward into evolution. It only explains why some variations are preserved instead of others. It does not explain evolution, but shows how the direction of evolution may be influenced by the environment. The causes of evolution, or, to be more explicit, the causes of evolutionary variations, are as mysterious to us as they were to Darwin, and indeed, more so, since the greatest step in evolutionary investigation since the time of Darwin has been a negative one, the destruction of the theory of the inheritance of characters acquired from the environment. Darwin sometimes placed much importance on variations induced by environment, and invented the theory of pangenesis to explain the inheritance of such, and bring them within the field of natural selection. Without pangenesis and direct inheritance, natural selection loses its place as a positive factor in evolution and becomes purely negative; it neither causes variations nor causes them to accumulate. The most that can be claimed is that it hastens the development of some characters by retarding others, or by forbidding them entirely. It is apparent in some groups of organisms that the influence of natural selection has been very great, in others that it has been very small,1 but its effects are in all cases dependent upon the underlying facts, that variations do appear and are accumulated. Natural selection does not explain evolution, except in a very loose and superficial  sense; the first step toward a better solution of the riddle is to reorganize the vocabulary of variations so that it can be used to express something more than erroneous deductions from natural selection. Many words and distinctions of use in presenting the idea that natural selection is a true, actuating cause of evolution, may be spared, but there are others whose utility is not destroyed by this change of view.
VARIATIONS AND INTRASPECIFIC DIFFERENCES.
Before entering upon a discussion, of a general scheme of variations it is necessary to notice a fundamental error commonly attached to the word variation itself. Most of the exponents of selective theories of evolution have made, either tacitly or avowedly, the assumption that all the individuals of a species are normally alike and tend to remain uniform, and that the differences found among them are of external origin and of the same nature as the differences between species, and hence of evolutionary significance. It has been assumed, in other words, that all the differences to be found among the members of a species are variations in the evolutionary sense, and hence that a cause of difference among the members of a species is necessarily a cause of the evolution of species. It is not too much to say that this assumption of normal specific stability and uniformity, either absolute or within constant limits, begs in advance the whole question of the nature and causes of evolutionary change. Notwithstanding the popularity it has enjoyed, this static idea of species is worthy of no more respect than any other unsupported hypothesis.
For the former purposes it appeared desirable to divide the variations, that is, the differences to be found among the individuals of a species, into two classes — (1) those with which they are endowed at birth, and (2) those which they acquired later from the external conditions of their existence. Variations were classified, in other words, as either congenital or acquired. The distinction is not illogical, but it has proved worse than useless for evolutionary purposes, because the static theory by which it was suggested was an erroneous assumption.
Many objections to natural selection, or to evolution as based  upon it, have been raised from the time of Darwin to the present day, but a doctrine with so many merits was not to be displaced until another could be found. Furthermore, the alternative views hitherto presented have shared either one or both of the false premises of natural selection, or they are built, like that theory, on some one group of biological phenomena, and leave out of account other data equally pertinent to the general conclusion, and equally in need of evolutionary explanation.
One of the ways in which the search for evolutionary causes went far afield was in assuming a close and essential relation between evolution and the origin of species. It was thought that if it could be known how new species came into existence the secret of the diversity of nature would be revealed. As a matter of fact evolution has very little to do with originating or multiplying species. The evolutionary process continues, we may believe, whether the group becomes divided or not. The two parts become different because evolution continues in both, but it would also have continued if the separation had not taken place. Isolation, of one kind or another, is the cause of the multiplication of species, but not of evolution. We would gain no special advantage for evolutionary observation by stationing ourselves at the point of bifurcation of one group into two; the only lesson would be that isolation isolates, that segregation segregates. Evolution, it cannot be repeated too often, does not take place in the gaps which are left between the species, but inside of the species, among the interbreeding organisms; it is an intraspecific phenomenon, not interspecific.
To learn how species differ is only to ascertain what roads they have traveled over, it is only by canvasing the differences between the individuals of a species that we can hope to ascertain how the evolutionary progress is accomplished. It will not suffice, when we find that the individuals of a species differ in a certain respect, to assume that this is the line of evolutionary advancement. We must be content first to recognize and describe the several kinds of intraspecific differences before we can hope to estimate with confidence the contribution of each form of change to the general and permanent progress of the species. 
CLASSIFICATION OF INTRASPECIFIC DIFFERENCES.
Intraspecific differences may be classified by reference to three considerations; the nature of the diversity, its origin or occurrence, and its relation to environmental fitness. Such a classification is open to the objection that it requires an advance decision upon the evolutionary bearings of the facts which are being classified for evolutionary purposes. This objection also applies, however, to all preceding efforts at classifying variations. Such classifications have no value, of course, as the basis of arguments. Their use is purely that of permitting an orderly arrangement of materials and of illustrating distinctions. They aid in discrimination, not in demonstration.
The utility of the proposed arrangement may be best appreciated by thinking of it, not as a classification, but as affording points of view or avenues of approach to the study of the intricate complexities of evolutionary problems. The purpose of physiological study is not classification, but the comprehension of causal relations.
Differences of Growth Stages.— Changes of size, form, structure, and function shown in the life‑history of normal members of the species, including metamorphosis and alternation of generations and structural phases. The forms of diversity grouped under this head would not be called variations except in the most general sense of the term, but they must be taken into account in making a complete outline of intraspecific differences.
Differences of Normal Descent (Heterism).— Individual and other differences, including those of sex and polymorphism, which appear among the members of the species under normal conditions of interbreeding in the same environment, and even among the simultaneous offspring of the same parents.
Differences of heterism have no relation to accommodational fitness, though they may assist in the evolution of adaptive characters. They have sometimes been called fortuitous or fluctuating variations because they had no apparent utility, the organic advantage of diversity of descent not having been recognized.
Differences of Accommodation to Environment (Artism).—  Differences resulting from the ability of individual organisms to adjust or accommodate themselves to different environments. These are the variations which have the most intimate connection with the environment, though they have no special significance as causes of evolution.
Differences of Deficient Accommodation (Topism).— Differences resulting from the inability of organisms to fully adjust themselves to special conditions. The result is a non‑hereditary divergence from the normal characters of the species.
Differences under New Conditions (Neotopism).— Variations induced by the transfer of organism to new and unwonted conditions. Three stages of new place effects may be distinguished, (1) those in which there is merely a stimulation of growth, (2) those in which there is also a definite mutative change of the hereditary characteristics of the variety, (3) those in which the new conditions call forth a promiscuous mutative diversity.
Differences of Partial or Recent Interruption of Interbreeding (Porrism).— Differences arising from the unequal distribution of variations, that is, from a recent or partial interruption of interbreeding. Such are the differences that exist between individuals from the remote parts of the range of a species (geographical differences) and the differences of segregated local varieties of domesticated species. The nature of these differences is the same as that of the differences between species. They are the result of divergent tendencies of evolution.
Differences of New Genetic Variations (Neism).— Prepotent variations which arise under normal conditions of free interbreeding, without having existed previously among the ancestors of the variant individuals. They can be preserved without isolation, and are the characters which probably contribute most to heterism, and to the normal evolutionary progress of species in nature. There is no evidence that the appearance of such variations has any connection with adjustment or environmental fitness. Their preservation depends, of course, upon their being useful, or at least not positively detrimental.
Differences of Aberrant Heredity (Teratism).— Failure of the organism to attain the normal form, structure or size of the  species. Teratism occurs whenever there is any accidental deviation from normal developmental processes, whenever conditions change beyond the practicable limits of normal adjustment, and whenever the specific network of descent is abnormally narrowed. Thus there are many kinds of teratisms, and many gradations between them and the other more normal kinds of variations.
Mutations are abnormal or teratic neisms which appear abruptly in inbred or narrowly segregated groups, and which require isolation in order to be preserved. Even when induced by changes of environment, mutations are to be reckoned as aberrations rather than as accommodations.
This classification makes no claim to final completeness, since still other kinds of intraspecific differences maybe discovered. No doubt the schedule will appear to some as already too extensive and complex, but it will be evident that none of the alleged kinds of differences can be left out of account without misinterpreting one or more of the other groups of phenomena. To overlook the facts of heterism would make hopeless confusion under artism, topism and neotopism. To fail to distinguish between neism and teratism is to mistake degenerative mutations for examples of progressive evolution.
Characters, in the morphological sense, cannot be classified and catalogued as heterisms, artisms, or teratisms. There is an intimate and even interchangeable relation between these different kinds of differences. An individual may be larger than others of its species, either as an inheritance or as a new variation, or because the conditions are favorable, or even because they are new. Finally its greater size may be abnormal, or of the nature of a monstrosity. The same character may thus have great diversity of evolutionary significance.
DIFFERENCES OF GROWTH‑STAGES.
Under this class of intraspecific differences it is proposed to include all the general forms and growth‑stages in which the members of a species normally appear in any part of their life history. Only in the lowest and most primitive groups do all the separate, individual organisms belonging to the same  species have even a general similarity of structure and external appearance.
There have been extensive and not altogether profitable discussions of the relation of growth‑characters to those of the adult and to the evolutionary history of the species. The older embryologists worked out a doctrine of recapitulation to explain larval and juvenile characters, but it is evident in some groups, such as the insects, that preliminary stages may be quite as adaptive as the adult form of the species, and sometimes distinctly more so. The differences of growth‑stages are themselves of very different types in the various natural groups, as a result of the great diversity of methods by which evolution has been accomplished.
THREE TYPES OF CELLULAR STRUCTURES.
|1Cook, O. F., and Swingle, W. T., 1906. Evolution of Cellular Structures. Bulletin 81, Bureau of Plant Industry, U. S. Department of Agriculture.|
The most fundamental diversity of form and structure which exists among the members of the same species is that which arises from the existence of different types of cell‑organization. In many of the lower groups of plants the vegetative organism, like a filamentous alga or a moss‑plant, is composed of simple cells which have not conjugated and which have in many cases no power of conjugation. In the higher types of plants and animals the body of the organism, in its highest and most complete form, is built up of cells in a double or conjugating condition. The higher fungi differ from the ferns, flowering plants, and higher animals in that the cells associate themselves while in the first stage of conjugation, before the nuclei have fused, while the cells of the other groups represent the second stage of conjugation. The nuclei have fused, but the chromatin granules still remain distinct.1
The great diversity of the cells which compose the bodies of the higher plants and animals may be viewed as a phenomenon of social organization. The lower the organism the more alike are the cells until in the lowest all cells are similar and equal. Where socialization, the habit of joining together or living in groups, has not progressed too far, the cells of compound individuals  may still be alike; the organization is still a mere gregarious association. Later, there may come about a division of labor among the cells, and a corresponding diversification of structure and form. The common pond‑scum (Spirogyra) consists of threads formed of cylindrical cells, joined end to end, and all alike in their vegetative and reproductive powers. Another similar organism (ődogonium) consists, for the most part, of similar chains of equal cells, but these have only vegetative functions. The power of reproduction has been restricted to two kinds of special sexual cells different from the vegetative cells.
Advance in the scale of organization not only maintained this distinction between the reproductive and vegetative cells, but continued to increase the numbers and differentiate the structures and functions of the latter, until the immensely complex bodies of the higher plants and animals had been built up.
The primitive type of cell organization, that which built up the filaments of the lower algae and the vegetative tissues of the liverworts and the mosses was not able, however, to reach the higher possibilities of cellular structure. The cells which compose the bodies of the higher fungi have two nuclei, and those of the flowering plants and higher animals have two sets of chromosomes. These double‑celled conditions have arisen through a lengthening out of the process of cell‑conjugation as it occurred in primitive types like ődogonium. Instead of conjugating at brief and distant intervals, the cells which compose the bodies of the higher plants and animals are in a condition of prolonged conjugation, the cell fusion which begins when the egg‑cell is fertilized by the sperm not being completed until after the whole compound cellular structure has been built.
Several groups of plants have two structural phases, one built of the primitive simple type of cells, the other of the double or sexual type. The moss‑spore, when it germinates, first produces a delicate tube like a pond‑scum, and the fern‑spore a small plate of simple cells, much like a liverwort. These diverse stages or phases of structure of the same organism have usually been described as alternation of generations, but the case is in reality entirely different from the phenomenon of alternation found among animals.
ALTERNATION OF GENERATIONS (METAGENESIS).
In many animals and plants the usual method of propagating new individuals by new sexual conjugations gives place to a more or less regular alternation with generations which are propagated vegetatively, or without a new conjugation. Among the animals, such as the tunicates and plant‑lice, the generations which propagated vegetatively have a form different from those which propagate by renewed conjugation.
Alternation of generations, in the proper sense of the words, occurs when the same species exists in two alternative forms, and especially where the two forms have different methods of propagation. The plant‑lice furnish the most familiar example of alternation of generations. We may suppose that, like other insects, they were confined originally to normal sexual reproduction, but their evolution has been in the direction of smaller size and simpler structure, and they have also developed the power of multiplying for several generations by parthenogenesis, the parthenogenetic generations being further distinguished by the absence of wings, and by being very short‑lived. At the end of the season winged insects of both sexes are produced, and normal fertilization and egg‑laying ensues.
No such alternation of sexual and parthenogenetic generations is known to have arisen among plants, though a similar interpretation might be placed upon the bamboos, for example, which propagate vegetatively by the branching of their root‑stocks for a long series of years. Then all the plants of the species blossom, bear fruit and die, at the same time. Each sterile shoot of the bamboo might be interpreted as parthenogenetic generation if compared with the sexually propagated generations of a plant like Indian corn.
Among the insects in particular, and to a somewhat less degree in many other animals (mollusca, crustacea, batrachia, fishes, etc.). pronounced changes of form and structure, sometimes very abrupt, take place during the life‑history of each individual. Thus caterpillars change by metamorphosis into butterflies, grubs into beetles, maggots into flies, tadpoles into frogs, etc. 
Metamorphic differences are largely adaptive, but it is none the less probable that the alternation of bodily forms and the change of food and environment may contribute something to the same physiological results as diversity of descent. In the more specialized insects metamorphosis is accompanied by a complete disorganization of the larval tissues, the pupae representing, as it were, a return to the egg stage, the change of external form affording an opportunity for a complete rebuilding of the cellular structure of the body. It may be that this fact, viewed in connection with the extremely complex nuclear organs of the cells of insects, will assist in explaining the unique efficiency of the insect organism.
Metamorphosis is not restricted, however, to animals. In plants like Eucalyptus and Juniperus there are sudden changes of form and structure from the juvenile to the adult phase of the species.
Many plant and animal parasites infest two or more hosts in different stages of their life‑history. Changes of hosts are then usually coincident with metamorphoses, or with change of generation or of structural phases. It has been inferred by some that the abrupt change in the organism is due to the change of food and other conditions of existence, but this does not find confirmation in the studies of the life‑histories of the parasites. The indications are more favorable to the opposite suggestion that the great diversity of conditions has enabled the parasites to proceed on two or more independent courses of evolution.
The parasites have developed the power of living in two or three distinct environments at different periods of their lifehistory, and the characters which adapt them to this variety of conditions have been attained, apparently, in quite the same manner as the characters of other less specialized plants and animals.
The more primitive simple‑celled stage, or haplogamic phase, of many species of rust‑fungi is confined to pines or to others of the more primitive families of plants, while the more advanced and efficient double‑celled phase of the parasite has been able to attack plants of more highly developed families,  such as the Leguminosae or Compositae. There can be little doubt in such cases that the evolution of the later phases of the parasites have taken place in coincidence with the advancing development of their host‑plants to which they are so strictly confined.
GROWTH SPECIALIZATIONS ARISING FROM SOCIAL ORGANIZATION (POLITISM).
Just as cells have become diverse by specialization in the building up of compound cellular structures, so individual organisms of the same species may become diverse under conditions of social organization, that is, when the individual organisms do not live singly and independently, but in groups, colonies or compound individuals. The bionomic unit of such species is no longer the individual but the colony, since it is only in the colony form that it meets its environmental problems or enters into relations with other species. A good illustration of politism is to be found among the compound types of higher plants, those which take the form of shrubs or trees and consist of aggregates of large numbers of the individual twigs or branches which correspond to whole individuals of simpler types.
The primitive herbaceous types of flowering plants have a root and a stem, the latter with a series of leaves and a flower at the top. If this be considered an individual, larger plants with many stems or branches and many flowers are compound individuals. Each branch or flowering twig of a tree may be thought of as corresponding to the small individual herb. Usually the branch‑individuals are all of one kind, or at least equivalent and able to replace each other, but in some species such as cacoa, coffee, cotton and the Central American rubber tree (Castilla) the branches are strictly dimorphic, that is, of two or more distinct kinds with different forms, structures and functions, and also taking definite positional relations in the building up of the compound individual plant or tree.
It is among the animals, however, that specializations of politism exist in vast variety, and the diversity becomes obvious and familiar. In many different groups there have grown up social organizations, so that all stages may be found between the  merely gregarious condition in which the individuals are still equal and alike, to those in which the diversity inside the same species may be greater than that of genera and families in other groups. In man himself social organization has scarcely gone farther than the gregarious state, though some races of mankind have more pronounced social instincts than others, and such instincts have undoubtedly been important factors in their progress or backwardness in civilization. In some countries distinct castes exist, but these are racial or historical in origin and scarcely amount to the attainment of intraspecific diversification.
By far the most compact and highly specialized forms of social organization are to be found among the insects. Remarkably similar conditions have been attained independently in several different families belonging to two very different orders, the termites and the hymenoptera. In these highly specialized insects the individuals of a species are no longer capable of independent existence, but, like the cells of the higher plants and animals, have no meaning except as parts of a collective, super‑individual organism. The nest or colony has become the true unit of the species, and its members are differentiated into numerous castes adapted to particular functions by pronounced differences of size and structure. Among the hymenoptera only the females have social instincts and take part in the labors of the nest or the hive, but among the termite both sexes are equally involved. Reproduction is restricted to a single royal pair, who do no work beyond burrowing in the ground after their first and only flight. The king and queen and their numerous progeny are fed and cared for, and the architectural and agricultural labors of the state are performed by hosts of sterile dwarfs, of which in some species there are as many as four different castessoldiers, foremen, workers and nurses, each distinct in form and highly specialized in instincts for its particular part in the labors of the city.
The body of the termite queen may be hundreds of times the size of that of a worker, and the head and mandibles of a soldier twenty times as large as those of a nurse. Termite communities often contain millions of inhabitants. They build structures far exceeding, proportionally, anything attempted by man, and  maintain underneath them immense systems of subterranean fungus gardens and chambers for storing and curing the cornminuted wood of which the gardens are built. This material is brought in from long distances by means of tunnels bored through the earth or covered passages built over rocks and tree trunks.
Politism is to be classed as a specialization of growth‑stages, because among the bees, at least, it has been found that the differentiation of the sterile worker from the fertile queen is determined by the amount and quantity of food given to the growing larva. It is difficult to believe, however, that this is true of the termites, for the young are not stationary grubs as among the bees, but active creatures which circulate to all parts of the nest, so that a consistent policy of feeding seems quite impracticable. Moreover, the workers and other sterile castes of the termites are not undeveloped females alone, as among the bees, but consist of stunted forms of both sexes.
DIVERSITY OF NORMAL DESCENT (HETERISM).
The individuals of a specific group may appear closely alike when compared with those of other species, but when compared with each other their diversity becomes obvious. Many evolutionary writers have believed in a principle of heredity which would make all the members of a species "exactly alike," and have then assumed that intraspecific diversity is due to variation of environmental experiences in one stage or another of the life‑history of the differing individuals. The kinetic theory depends upon neither of these hypotheses, but recognizes the diversity of individuals inside the species as a normal and highly significant evolutionary phenomenon, for which the term heterism has been proposed. Plants and animals propagated under the same conditions may appear more similar than others of the same stock grown under diverse conditions, but they do not tend to any complete uniformity except as this is brought about by the abnormal inbreeding to which domesticated varieties are usually subjected.
Heterism might be defined further as the morphological aspect of symbasis. To support and hold together the organic  structure there must be an interweaving of lines of descent among diverse individuals. This requirement is most conspicuously met by the familiar phenomena of sex‑differentiation, but can be traced upward through all the intermediate stages from simple heterism, or mere individual diversity.
As manifestations of heterism are to be included all stages of intraspecific diversity, from individual differences to the extreme specializations of the sexes and polymorphic forms of the higher plants and animals. The function of heterism is to afford diversity of descent, under conditions of symbasic interbreeding. Narrow segregation or selective inbreeding tends to eliminate heterism, but with the inevitable result of degeneration. Heteric characters are highly heritable and though sometimes affected by environmental conditions are in no way dependent upon them or caused by them.
Purity of stock and unformity of characters are not synonymous terms, as commonly supposed. A very "pure" inbred strain may degenerate and become inconstant through mutation, or there may be the diversity of dimorphism or polymorphism in a species or variety which has not been crossed with any alien blood.
Heterism, in its most general and unspecialized sense, is what has been called by some authors individual variation or fluctuating variation. It includes the regular and normal individual diversity of the memhers of a species which is not induced by differences of external conditions. Some writers do not admit that there is any such diversity, not caused by external conditions.
It is very difficult, of course, to say that any given character or difference may not be connected with an environmental change, but it is very easy to ascertain with reference to most of the so‑called individual differences, that the environmental relation, if any, is not at all constant, and not to be established on the basis of any form of scientific observation yet suggested. We are perfectly aware that the children of the same parents, born and raised under the same roof are often very unlike, while on the other hand, close family likeness may persist between children born and bred in remote parts of the earth involving the completest possible change of climate, food, and other conditions of existence. 
Intraspecific differences, or variations, as they have been called, have been interpreted hitherto either as results of environmental influences or as steps toward evolutionary change. The recognition of heterism, or the diversity of normal symbasic descent, is incidental to a third explanation of the value of variations, that they help to maintain the vital strength or organic efficiency of the species.
Indeed, the frequency and extent of the differences of sexes, castes, races and alternating generations show not only that organisms may change without being divided into separate species, but also that diversity inside the species has an evolutionary as well as an environmental significance.
Heterism has, if this suggestion be well founded, a concrete physiological value in the economy of the species, quite as real as food and water, though of a different kind. The fuel and water are necessary to keep the engine going, but it is also necessary that the machine be kept in repair and from time to time replaced by another built on the same plan.
Environmental variability or power of accommodation, enables the species to operate under a variety of external conditions, but heteric variability provides diversity of descent, even under uniform and favorable conditions, and thus makes it possible for the species to continue to produce new individual organisms as good or better than the old.
Theories of evolution by environmental causation have overlooked heterism and have assumed that the individual members of species would be alike if there were no environmental inequalities to make them different. This assumption is contrary, however, to all the pertinent facts observable in nature. Acquaintance with the members of any wild species of plants or animals soon shows that individual differences exist, as great, and often greater, than those recognized everywhere among men and women, or among horses, dogs, tulips, roses, grapevines or apple trees. Definite individual diversity, as of stature, features, and thumb marks is not confined to the European races, nor to the human species. Travellers newly arrived in Africa or China often have the impression that the natives are all closely alike, but with longer residence they appear as different as Europeans. 
Likewise with plants and animals; it is necessary only to become personally acquainted with them to appreciate their individual differences. The shepherd knows all his sheep as individuals, also the poultry‑raiser knows the eggs of the individual hens, and the farm boy knows the kind of nuts which each hickory tree produces.
An instructive instance of natural heterism was observed in a species of agave which is extremely abundant on the mountains to the north of Chiantla, in the department of Huehuetenango, Guatemala. The size, shape, color and spine‑development of plants growing by the hundreds along the roadside varied endlessly. Some were pale‑green and heavily pruinose, some slightly pruinose and much darker green. Some tapered rather gradually to the point, some carried their width to near the end. On some the spines were very numerous and prominent, on others scattering and small, and with all grades and combinations of these and other varying characters. It is not claimed that these agaves have essentially greater individual differences than other plants. The phenomenon of heterism is rendered unusually striking because their large leaves have a very definite form and are closely alike on the same plant, and thus give unusually favorable opportunities for observing and comparing the differences which exist.
SPECIALIZATIONS OF HETERISM.
The recognition of the facts of heterism, the existence of intraspecific diversity for its own sake, and of its own physiological value to the species might appear to rest on merely theoretical ground were it not for the many specializations of heterism for which no use or meaning has even been imagined, other than that of maintaining a desirable diversity of descent.
In some species heterism has remained unspecialized. The individuals are different, but still all equivalent and alike, possessing all the essential vegetative and reproductive parts. Such species secure the benefits of heterism only by the introduction of new characters, for each character can be shared ultimately by all the members of the species and thus ceases to be of value as a means of maintaining diversity of descent. 
Heterism becomes specialized when there are permanently established differences among the members of the species, as in the familiar phenomenon of sex. There is also a series of many gradations between unspecialized heterism of merely individual differences, and the fully established sex‑differentiation. The separate sexes of the higher animals are so familiar a phenomenon that we have been satisfied to consider them merely as incidental to the process of reproduction, and have thus overlooked the additional physiological value of sexual differences as specializations of heterism, to insure diversity of descent.
In man himself and the higher mammals and birds the principle of sexual selection enunciated by Darwin may have had an influence in the further accentuation of sexual differences such as beards, wattles, combs, tail‑feathers and other means of rendering one sex or the other conspicuous and thus attracting their mates, but secondary sexual differences are not confined to the higher groups or even to animals. Many plants are unisexual and the two sexes often have differences other than those of the essential organs. As the two sexes of plants neither see nor come near each other, the pollen being carried by the wind or by insects, there can be no question of sexual selection here. Even types as lowly as the mosses and liverworts often have the sexes separate and very unlike. Nature furnishes, indeed, hundreds and thousands of instances of independently acquired sexual diversity without use either in environmental relations or in reproductive processes.
The use lies, we may believe, not in the particular differences but in the diversity of descent which the species is enabled to maintain. Diversity is of value to a species not only to enable it to exist under a variety of conditions, but also because diversity in descent is an important factor in maintaining the organic strength or vital efficiency of the individual organisms. We may still believe that all character differences have their uses, but the use is not confined to environmental or selective considerations. More fundamental than these is the use of the diversity to the organisms themselves.
Sexual differences contribute, in other words, to the increased effectiveness of sexual reproduction, that is, they intensify the  effects of fertilization or cell‑conjugation in endowing the new organism with the power of vigorous growth. With this interpretation of sexual differences in mind we are the more ready to entertain the idea that specializations of heterism would be beneficial, even apart from the sexual diversification of the species, and are thus able to recognize and appreciate a group of phenomena which has hitherto remained meaningless and neglected.
Since the time of Sprengel and especially since Darwin, it has been known that many plants, even those which are bisexual, or provided with both pollen and egg‑cells, have many specialized habits and devices which serve to secure cross‑fertilization. Although possessed of pollen of their own the flowers are often so formed that the pistils receive pollen only from abroad, and in many species foreign pollen is a necessity, pollen from the same plant being entirely ineffective. The advantage of cross‑fertilization being admitted, the value of these adaptations for securing it becomes obvious, but the benefits lie, as Darwin discovered, not in the "crossing by itself" which "does no good," but in the diversity of parentage which may in this way be brought about. These specializations have, in other words, a double function; they assist in the crossing and also minister to the diversity of descent which is the object of the crossing. They have, in other words, the same function as sexuality, and have been interpreted by naturalists as a simple or incipient form of sexuality.
Still simpler specializations of heterism have only one of these two functions, that of maintaining the diversity, but without assisting in the bringing of the diverse parents together. The crossing is left, apparently, to chance, but when it takes place the diversity renders it the more effective. As instances of this simple type of specialized heterism may be cited such species as Verbascum blattaria, the flowers of which are pink on some plants and yellow on others. The two types grow freely intermingled over wide ranges of country but no intermediates are found. 
DIFFERENCES OF ADJUSTMENT TO ENVIRONMENT (ARTISM).
The notion that all of the differences to be found among the individual members of species are caused by inequalities of environmental experience finds no warrant in the vast mass of experimental facts accumulated by agricultural experience with domesticated plants and animals, nor in observations of species in undisturbed natural conditions. The differences which can be ascribed directly to environmental influences are relatively few and of little importance for evolutionary purposes. Of indirect effects of environment there are two principal classes, those which arise from the ability of organisms to adjust or accommodate themselves to different environments, and those which result from a disturbance of heredity by new and unaccustomed conditions.
The individual members of species often differ among themselves as a result of the possession of a certain range of organic elasticity or power of adjustment to different environmental conditions. Such differences are commonly greater among plants than among animals, for the latter are often able, through the power of locomotion, to choose or to control the conditions under which they shall exist, while stationary plants are subject to much wider ranges of environmental vicissitudes. It has often been taken for granted that these differences of accommodation are direct results of environmental influences, the organism being thought of as having a merely passive plasticity. The fact is, however, that this power of accommodation is as positive a phenomenon, as truly a form of organic activity, as growth, locomotion or reproduction, and as worthy of a definite and appropriate designation in evolutionary literature.
Indeed it is no mere figure of speech to term these differences accommodations. The word can be used of plants and animals in their environmental relations in quite the same sense as for the change of convexity executed by the human eye to enable objects to be clearly seen at shorter or longer distances.
This group of intraspecific differences has received a large amount of study from evolutionary specialists, and especially from ecologists and others who hoped to find the causes of evolutionary  progress in mechanical effects of environmental influences. A large number of special phenomena of artism have been named, such as heliotropism, or the power of plants to grow toward the light or to turn themselves to face the sun. Geotopism is the opposite tendency of the roots to bury themselves in the soil.
Some writers on "evolutionary mechanics" have gone so far as to name the tendency of birds to stand or fly facing the wind as pneumotropism, and of fish to head up stream as rheotropism. Consistent prosecution of this tendency to ascribe special "forces," and to give technical names to each habit or instinctive act could result only in confusion, worse, indeed, than the older practice of ascribing all unexplained organic phenomena to a general "vital force." Even the operations of agriculture are conducted by many primitive peoples on an instinctive rather than a rational basis. In spite of permanent employment and a fully assured supply of food, the Indians of Central America obey an internal compulsion to scatter upon the land, when the proper season comes, to clear and plant their corn fields. Owners of mines and plantations have reconciled themselves to a complete suspension of work during the cornplanting weeks, having learned by experience that it is useless to oppose or to reason with this irresistible agricultural impulse.
It would be possible, of course, to describe this agricultural instinct as a form of geotropism, a turning to the land for food as the root turns to the soil. The practical point is not, however, the choice or application of terms, but to note the probability that the instinctive actions by which man and the higher animals adapt themselves to environmental needs belong to the same general class of phenomena as the accommodative changes of plants. We know why we clear the land and plant our crops, and if the need or the advantage be not present we have no difficulty in discontinuing our agricultural labors, but it is not likely that agriculture arose, in the first place, as a conscious and deliberate art. Its beginnings are probably to be traced back by imperceptible stages to the primitive root crops of tropical America which grow readily from cuttings of the stems and rootstocks, so that the digging and harvesting of one crop plants and cultivates the next. 
We permit ourselves to say that agriculture was learned in some such accidental way, but we forbear to say that plants also learn to adapt themselves to take better and better advantage of environmental requirements. We base the distinction on the fact that we have reasons for our actions, but in the great majority of comparable cases the reasons have been discovered long after the arts had been perfected. We have theories of swimming, but young children often swim quite as instinctively as animals.
This may appear an entirely irrelevant digression, but a useful purpose may have been served if we are ready to recognize the essential unity of the phenomena of accommodation or direct adaptation and cease to demand special explanatory terms and hypothetical forces for each of the multifarious forms of adaptive change. The explanation will come when our knowledge of protoplasmic organization has sufficiently increased, but in the meantime we gain nothing by multiplying the mystery or by giving it a multitude of names.
Under the theory that environment causes evolution a very real and important relation was supposed to exist between artisms, or adaptive alternative characters inside species, and ecology, or the study of the adaptive characters of species.
Artisms or environmental adjustment variations have received much consideration from those who have held that evolution is caused by the environment, and who have believed, in accordance with this view, that the environmental variations were true examples of progressive evolutionary change, carried forward by external influences.
This doctrine became untenable when Weismann showed that characters directly "acquired" from the environment are not inherited, that is, they do not show any tendency to repeat themselves unless the inducing conditions are present. Weismann proposed to explain the possession by the same species of alternative characters by his theory of determinants, or internal "mechanisms of heredity." These determinants were thought to control in advance the characters of the organism, and alternative characters were explained as the work of two or more sets of determinants which could be brought into action by particular  conditions. Where the alternatives are sharply defined as in the two sexes of man and the higher animals this theory might appear to be applicable, but where, as in many plants, there are, even in the same species, all stages of sexual differentiation, or many distinct castes or forms, with or without reference to the sexes, the theory of determinants becomes impracticably complex.
In the experiments of Standfuss with butterflies it has been found possible, by changes in the temperatures in which the pupae are kept, to influence the colors of the adults so as to approximate those of a different geographical variety or seasonal form. It has been inferred as a consequence that temperature is a direct evolutionary factor in causing one species to change into another. In reality, however, this is but one of the many instances in which failure to distinguish between the taxonomic and the evolutionary standpoints has permitted confusion to enter. Some of these seasonal and geographical forms of butterflies have been named as distinct species, but if it be found that the supposedly distinctive characters are merely artisms or accommodations to temperature, the proper step is to revise our classification before attempting to use it as a basis of evolutionary inferences. The largest possibility suggested in the present instance is that abnormal temperatures may induce in one part of a species a character which another part has reached by normal evolutionary process. The fact that the different geographical color races may have been described and named as species and varieties cannot be made to prove that temperature is a cause of species‑formation.
This power of accommodation to the environment, specific elasticity or artism, may be thought of for evolutionary purposes as a general character of the species, but like other characters it is possessed in different degrees by different individuals, and this difference of degree is as heritable as any other feature. Some individuals and strains of a species may have greater range of elasticity on both ends of the series, while others have greater freedom of change in one direction than in the other, for example, they can become very hairy, but not very smooth. Still again, we find mutative variations toward a restriction of the normal  range of development. Some of the coffee mutants have extremely short internodes. None of these complications need obscure the fact that the phenomena of artism can be viewed as entirely distinct from those of heterism, though neither phenomenon excludes the other.
DIFFERENCES OF USE AND DISUSE.
One of the reasons for the persistence of the belief that adjustments to external conditions represent direct effects of environment, lies in the fact that several other kinds of intraspecific differences have been confused with environmental adjustments. Most of these additional types of diversity are rather uncommon, but they are well calculated to confuse thought and even to vitiate experiments, especially when these are undertaken without fully considering all the sources of possible error.
If an animal or a plant be kept in captivity or placed otherwise under conditions where its normal activities are not called into use, muscles or other organs may fail to reach their normal development, or they may actually decline in size and deteriorate in structure under continued disuse. There are certain senses, of course, in which it may be said that the environment, by determining the use of parts, causes them to prosper or decline, but closer attention will show that these are phenomena of growth and nutrition rather than of environmental adjustment. The use of a muscle is as truly a condition of its development as the food from which the tissue is nourished, and the decline of such a part may be reckoned as a starvation phenomenon, or interference with the normal processes of growth.
The fact that so much has to be learned through precept and practice by the young of the human species has led some to overlook the existence of definite instincts and muscles which develop without use, just as the internal organs and functions develop in the embryo before birth.
The idea that there is a natural and general tendency to evolutionary motion, to change of organic form and structure, need not be confused with the predication of a principle of evolutionary perfection by which some writers have thought that  organisms might be carried along in an ever‑upward direction. Some species have gone forward or upward, but for each of the groups which has been able to perpetuate itself by continuing upward there have been hundreds and thousands which have not continued in lines of effective progress, but have turned aside and have been extinguished. This is as true of man and of human societies as of species. They do not tend to go upward but they do tend to change and these changes have carried a few upward to higher levels, where new planes of development and expansion were possible, but where the probabilities of still further steps were as doubtful as before, and as truly dependent upon correct, if unconscious choice. One view is teleological, the other purely causational.
The phenomenon of degeneration, the reduction or elimination of unused parts or organs, has led to the placing of undue emphasis upon the utilitarian aspect of evolution. Darwin attempted to connect the deficient size and strength of the unused organs of the individual with their reduction in the species by means of his theory of pangenesis which assumed that all parts of the body contribute to the reproductive cells. Degeneration was made a converse of natural selection; the reduction was believed to appear first in the adult, and then the negative acquired character was transmitted to the next generation. Many characters of adult organisms consist in part of a genetic or hereditary contribution, which might be called a qualitative element, to which is added during growth a quantitative reaction to more or less favorable conditions, depending not only upon external circumstances but also upon the perfection and efficiency of the remainder of the organism. Disuse undoubtedly affects the quantitative side of the development of voluntary muscles and other analogous organs, but it is not easy to understand how a progressive reduction could be brought about on Darwin's hypothesis.
After the elimination of the quantitative element due to use, a state of stability might be expected to ensue, unless there be predicated in addition a principle of organic economy tending to the gradual and continued elimination of useless characters and organs. In other words, the effect of pangenesis acting  alone would be limited to comparatively few generations, and would dispose of superficial and recently acquired characters only, an inference apparently supported by the persistence of many rudimentary organs.
The extreme constancy of vestigial characters confirms the a priori expectation that selection would have little to do with them except to eliminate; but differences, nevertheless, occur, of which progressive modification without selective influence must necessarily be predicated.
Weismann's panmixia was intended to represent a view diametrically opposite to that of Darwin, approaching the question of reduction from the side of heredity only, and laid emphasis on the opinion that, selection being discontinued, indiscriminate crossing without reference to the character previously at a premium would result ultimately in the reduction of the selectively developed parts. But even if it be admitted that a reduced average would be attained within specific limits or where intercrossing is possible, panmixia remains entirely inadequate to explain the progressive elimination of wings, legs, eyes or other important parts of the body, unless it be extended, as in the previous case, to an organic law of economy, a proposition logically quite distinct from panmixia. It is of incidental interest to note that both Darwin and Weismann have thus tacitly admitted a law of organic motion in the direction of the simplification of organisms, and that this proposition is again the exact opposite of that of Nägeli whose "Vervollkommungsprincip" works from the simple to the complex.
The phenomena of degeneration may appear to militate against the idea of a spontaneous organic motion. The belief has been that though organisms are in a sense elastic, in that one or more characters can be far drawn out by selection, they tend more or less promptly to return to what might be viewed as the previous condition of rest or equilibrium. Especially would this be the case where selection has been very acute and has accentuated one character at the expense of the total efficiency of the organism with reference to conditions other than that which has determined the special selection. The removal of the latter would then involve the loss of the advantage gained by selective  response to the special demands. In groups subjected to an active struggle for existence this would mean a change of direction rather than a cessation of selection. In many other instances, notably among parasitic forms, the loss of normal organs ascribed to disuse is better explainable by selection, since the apparent degeneration is of decided advantage from the standpoint of the actual life‑history of the animals.
The principle of panmixia seems, indeed, to involve an unwarrantable extension of the idea of organic elasticity, since it implies that organic structure is maintained by selection alone, without which everything would drop back to simple protoplasm. Of such a general tendency to degeneration there is, however, no indication. As explained elsewhere, the reversion of inbred highly selected types to the wild form of the species is not degeneration, but a recovery of normal structure after restoration to normal conditions of interbreeding.
DIFFERENCES OF DEFICIENT ACCOMMODATION (TOPISM).
Environmental differences are not all of one kind. Some of them are the results of the power of accommodation or adjustment (artism), while others represent rather a deficiency in ability of this kind, so that the organism, though perhaps able to maintain an existence, fails to attain one or another of the normal characters of the species. Thus there is a variety of canary bird which if fed on cayenne pepper during its period of moulting produces red feathers instead of yellow.
The South American Indians are said to be able to alter the color of the feathers of their domesticated parrots by inoculating them with the blood of toads. The colors of certain flowers can be modified by special conditions or by treatment with chemicals. The injury of the white pigs from paint‑root, while black pigs escaped, as related by Darwin, would be another example of the same group of phenomena.
The relations of topism to artism and to teratism are sometimes very intimate. A character assumed by one plant as a means of accommodation may appear in another as a limitation of the power of accommodation or as a complete abnormality. The need of discrimination and the difficulty of exercising it  are frequently apparent in the literature of the subject. Thus it has been inferred from experiments on a spiny New Zealand plant that the spines, instead of being a means of protection against grazing animals, of which there were none in New Zealand, are in reality an adaptation against transpiration, because they do not appear when the plants are cultivated in a humid atmosphere.
"After being placed in the moist chamber, the plants developed no more spines and are now seedling plants in all respects except for the few spines, which were developed prior to the culture in moist air. Moreover, it seems evident that such plants would remain in the seedling form so long as they were kept in an atmosphere constantly moist and exposed to a feeble light.
"Even an adult shoot on a full grown plant in the open and freely producing spines, may have any further production of such suppressed at once, if the shoot should continue its growth under slightly more hygrophytic conditions. Thus quite recently, I observed on the clay hills near Wellington, a shoot creeping near the ground whose apical portion was covered by grass. This shoot where fully exposed to the light was spinous as usual, but where shaded and in a slightly moister atmosphere was quite without spines.
"From the above it follows that the production of spines in Discaria Toumatou can be controlled at will by specifically changing its environmenta plant exposed to a dry atmosphere and normal light producing spines, whilst one exposed to a moist atmosphere and a feeble light produces no spines, but in their place leafy shoots of unlimited growth.
"That spines on xerophytic plants are an adaptation against the attacks of grazing animals is a matter of such general belief as to be admitted into certain botanical text‑books as a proved fact.
|1Cockayne, L., 1905. Significance of Spines in Discaria Toumatou Raoul (Rhamnaceae), New Phytologist, 4: 79.|
"It seems, however, to me that my experiment, detailed above, is a fairly crucial case, and that in Discaria Toumatou, at any rate, the spines are a direct response to conditions of dryness, and function as a special contrivance for checking transpiration. If so, then they have nothing to do primarily with attacks of  grazing animals, especially when it is borne in mind that New Zealand never contained such, excepting the various species of Moa."1
That the spines did not develop under conditions of moisture and feeble light can scarcely be accepted, however, as proving that they are a special contrivance for checking transpiration, for many analogous adaptations do not fail to appear in advance of the conditions which require them. Cacti, and other spiny plants often make most of their growth in periods of humid weather, but they do not on that account fail to put on spines.
The possibility that the spines may be a useful form of tissue for the plant when living in the normal desert habitat is not a sufficient explanation of the failure to produce the spines under conditions of humidity and deficient sunlight. The spines might be an adaptive character and still appear under all conditions of growth. They might represent an adjustment character or artism and still be only reduced instead of being eliminated in the shade form. That the spines disappear entirely indicates that another factor may need to be recognized, that certain conditions are necessary for their development, and that without these conditions the plant is unable to make spines, just as the pepper‑fed canary birds may be thought of as no longer able to produce yellow feathers.
The interest of the Discaria experiment would have been increased if it had included a test of the behavior of the plants in shade conditions without excessive atmospheric moisture, to determine whether deficiency of light might not of itself inhibit the formation of the spines, simply by restricting the activity of the cells. The formation of the spines is a specialization which the seedling plants do not attain until they have grown to considerable size, perhaps not until they have encountered conditions of drought and exposure to strong sunlight. It is, therefore, not unreasonable to suppose that these conditions are a necessity to enable the plant to produce the spines, and hence that its failure to produce them represents not so much an accommodation as a lack of accommodation, that is, topism, instead of artism.
1Lydekker, R., 1904. The Field, 104: 654.
 The prompt loss of wool by sheep brought to tropical countries is one of the most striking instances of response to environmental conditions, but there are several elements which need to be taken into account in attempting to arrive at a clear understanding of the nature of the process. The continuous heat and excessive humidity may induce an abnormal condition of the skin and cause the hair to fall out, as often happens in human fever‑patients. On the other hand, the failure of the sheep raised in the tropics to produce wool may be due to a lack of sufficiently normal conditions of existence which disturbs the normal heredity and affects first the most highly specialized character of the animal. The loss of wool could be explained in this way as a deterioration or reversion rather than as a new or adaptive character. The domestic sheep is now supposed by Lydekker to be descended from wild types which had a hairy summer coat and produced wool only as cold weather approached.1
|2Apples, cherries and many other temperate trees and cultivated plants fail to reach productive maturity under consistently tropical conditions, just as the seeds of lettuce may refuse to sprout without alternations of temperature, and the eggs of some mosquitoes refuse to hatch unless they have been frozen.|
Many animals and plants require the seasonal vicissitudes of heat and cold as a normal part of the conditions of existence, and refuse to behave normally in tropical regions where wide ranges of temperature do not occur.2 Indeed, the changes of temperature appear to supply to some of them the same kind of bodily vigor to which diversity of descent contributes. The plants and animals of tropical regions appear to have relatively great rapidity of evolutionary progress, as pointed out by President Jordan, who finds that the tropical fishes are much more highly specialized than those of extratropical waters.
|3Jordan, D. S., 1901.
Science, N. S., 14: 566.
"The processes of specific change, through natural selection or other causes, if other causes exist, take place most rapidly there and produce most far‑reaching modifications."3
It has not been shown, however, that natural selection is less acute in the colder regions of the globe; in fact, the general impression has been that the requirements are the more stringent and exacting. 
TEMPORARY EFFECTS OF NEW CONDITIONS (NEOTOPISM).
Experiments to test the effects of different environments upon plants are often interfered with by a temporary stimulation of growth, due, apparently, to the fact that the conditions are new, rather than to any essential superiority of the new place.
Like travelers in foreign countries they may often behave in a manner very different from their habits at home. Organisms, as well as men, though not built by their environments, are often built into them to such a degree that where the accustomed supports and restrictions are taken away the usual courses of action are no longer followed. New and unexpected characteristics assert themselves, not only or chiefly because the new conditions cause the organism to vary, but because they give it an opportunity to do so, or strengthen and bring to expression some tendency or instability of equilibrium. The new characteristics which have a definite connection with the new environment and are in the nature of adjustments to it may be expected to continue, but there is, in addition, a temporary effect, a temporary lack of adjustment, or a stimulation or aberration which sooner or later disappears.
This phenomenon may be called neotopism, or the new place effect. It is often strikingly shown in plants, and is not lacking in animals. The most familiar example of it is, perhaps, that of the tonic medicines. A vast number of substances, utterly unlike among themselves and having utterly diverse specific actions upon the human system when taken in large quantity, may nevertheless produce the same beneficial effect of temporarily increasing the efficiency of the organism, when taken in extremely small doses.
Neotopism is also to be reckoned as one of the factors contributing to the great vigor and rapid distribution of plants and animals immediately following their introduction into a new region. It is true that they may also have the advantage of immunity from diseases or natural enemies to which they were subject at home, but this is by no means a sufficient explanation of the unusual vigor and fecundity which they manifest for a time and which disappears after a series of years. Many plants, like the Russian thistle, which terrified the agricultural regions  of the Middle West a decade ago, after threatening for a time to become permanently injurious pests, have taken their places as comparatively peaceful settlers among the older plant inhabitants.
Neotopism is a phenomenon long known in practical agriculture, but hitherto not explained and generally not accepted in the scientific world, because the requisite evolutionary viewpoint was lacking. Having come to appreciate the physiological functions of heterism in maintaining the vital efficiency of organisms, we are in position to understand that a transfer to new conditions may also act as a direct stimulant of organic vigor, an artificial symbasis, as it were, which has probably contributed much to the sustained vitality of our inbred cultivated plants.
Likewise the heterism of the species might be thought of as increased by the extension to the new locality, and the added neotopic diversity might serve the same purpose as normal heterism in helping to maintain the organic vigor of the species as a whole, under conditions of free interbreeding. Thus devices for securing wide distribution serve the interests of the species in a variety of ways. They not only tend to increase the numerical prosperity of the group, but increase the facilities for interbreeding among the members of the species and also give it the benefit of as widely different conditions as possible. The diversity of conditions accentuates diversity of descent and thus contributes to the vigor of the species. With sedentary plants in particular we should be prepared to learn that changes of conditions of growth are as beneficial as changes of diet for man and the higher animals.
In many crops it has become a regular agricultural practice to exchange seed between more or less distant localities. Seed planted in a new locality often produces better and more fertile plants than in the place where it was grown, and better than the same stock after it has been planted in the same place for a series of years. The new conditions afford, for a time, the same physiological benefits as diversity of descent and new variations, and constitute, indeed, a striking confirmation of the physiological relations of these groups of phenomena.
In many other cases neotopism may only bring to the surface  and accentuate conditions of degeneration. Many varieties of domesticated plants and animals have been bred so long and so narrowly in one particular locality that any change is accompanied by notable deterioration. Thus it comes to be believed that seeds of one particular plant, such as the radish or the cauliflower, can be grown to perfection only at Erfurt. Transferred to any other point, neotopic mutation at once appears and brings diversity and commercial inferiority. In a similar way many high‑bred animals like the Jersey cattle also deteriorate or show special susceptibility to disease when subjected to new conditions, even to those in which other less closely adjusted breeds are able to thrive.
BEARING OF NEOTOPISM UPON ACCLIMATIZATION.
Neotopism must also be taken into account in another department of agricultural investigation. The phenomenon is often very marked in plants introduced from tropical countries into temperate regions, and has had the opposite effect of deceiving us regarding the possibility of acclimatizing species or varieties of tropical origin. The popular impression is that the colder climate of our more northern latitudes will restrict the growth of plants from the tropics, but this is the reverse of what usually happens, as a matter of fact. It seems to be a general law that annual‑crop plants, whether of temperate or of tropical origin, are most vigorous and productive near their northern limit of growth. The reason for this is that the longer days supply a greater amount of heat and sunlight than in the tropics themselves.
Plants newly introduced from the tropics commonly misuse these exceptionally favorable conditions to put forth an abnormal amount of vegetative growth and are often killed by frost before they commence fruiting. It has been usual to explain the failure of such experiments on the simple ground that our northern season has proved too short for these tropical varieties, but as a matter of fact the time may have been equal to that required by these same varieties for normal growth and maturity at home in the tropics. Thus the Kekchi variety of Upland cotton, which matures seeds in Eastern Guatemala in five months from planting, required in Texas over six months to produce  a much smaller crop the first year after its introduction, and might have produced no seed at all if the tendency to abnormal luxuriance of growth had not been checked by a long period of dry weather. Other tropical varieties of cotton have consistently refused to produce seed when introduced into Texas, even though the same length of season would have been sufficient in their home localities.
With the superior conditions of growth supplied by our northern summers most of the tropical varieties would be able, if they utilized their opportunities properly, to develop even more rapidly than they do in the tropics, and this result has been reached with some of the Mexican varieties of corn. During their first seasons in the United States they became greatly overgrown and ripened scarcely any seed, but after a few years they recovered their short‑season qualities and became especially useful as extra‑early varieties, like the "Mexican June" corn.
The conditions under which such experiments are usually made are well calculated to intensify neotopism instead of holding it in check. It has been reasoned after the analogy of our domestic varieties that fertile soil and thorough cultivation will conduce to the early maturity so much desired. Moreover, it is the regular practice to keep testing gardens and experimental plots in the best of condition. The result is that the newly introduced tropical variety is surfeited with the unwonted supply of readily available food and moisture, which still further increases the tendency to abnormal vegetative growth.
Many such varieties have entirely failed of acclimatization because they ripened no seed at all in the localities in which the first experiment happened to have been made. Nevertheless, the inference is not warranted that such varieties cannot be acclimatized in temperate regions. Experiments in the introduction of new types of Upland cotton from Guatemala have shown that the tendency to rank and sterile vegetative development can be controlled by carrying the new stock far enough to the north and placing it in comparatively sterile soil. In the latitude of Washington the Guatemalan varieties of cotton showed much more normal habits of growth, and made more  progress toward fertility and seed‑production than in the much longer growing season of Texas. These experiments afford a definite intimation, to say the least, that by the proper choice of conditions for the first planting the neotopic stimulation of tropical varieties can be held sufficiently in check to permit the maturing of at least small amounts of seed. This opens the way to the practical acclimatization in the United States of useful varieties of cotton, corn and other important food‑plants of tropical origin.
Further experiments have shown that the second generation of cotton in the United States is notably earlier and more productive than the first generation, when grown from seed of the same origin and planted in adjacent rows. It has also become evident that there are at least three stages or kinds of new place effects to be considered in the acclimatization of different varieties and types of cotton. The changes of hereditary behavior which can be induced by the transfer to new conditions are not limited merely to increased size or vigor, but have obvious bearing upon the phenomena of mutation, since the plants may change in a very definite manner in characters which would usually be considered of varietal or even of specific importance. The lack of fertility which accompanies the aberration from normal characters affords a further analogy with mutations. Nor does the interest of the experiment end here, for it has been proved that this neotopic form of mutation may supervene in a perfectly definite manner even after the plants have grown for a time according to the specifications of normal form and habits of the variety.
|1White, C. A., 1905. The Mutations of Lycopersicum, Popular Science Monthly, 47: 151.|
When the change takes place early the whole plant may show the abnormal characters and may be more or less completely sterile. In another locality plants of the same origin may grow for a time in a normal manner and remain normally productive, but may then change suddenly and completely to the abnormal, infertile, neotopic condition. In this form of neotopism the behavior of the individual plants grown from the same lot of imported seed is often remarkably uniform and the result is closely parallel to that described a few years ago by Dr. C. A. White in tomatoes. Two lots of seed produced, with much  uniformity, progeny so unlike their parents that Dr. White described and named them as a new species.1
A third result sometimes reached by transferring plants to new conditions is to induce a more or less general outbreak of miscellaneous variations of an abruptly mutative character. In such instances the stimulation effect may be lacking or very inconstant. Some individuals may be several times as large as their parents, while others are as much smaller.
Although the new conditions evidently induce the mutative variations, they can not be said to cause them, in any definite evolutionary sense, as proved by the great diversity of the mutations which the same change of conditions may call forth. The unfavorable conditions unbalance the organisms, but the individual lapses from normal heredity take many different directions, without reference to particular requirements of the environment.
The practical significance of the new‑place‑effects is, therefore, entirely different in different instances. As long as the result is an increase of vigor and fertility, the phenomenon is a useful one; but if the stimulation be so great as to change the characters of the plants and render them infertile the crop may be ruined, and this misfortune may also be reached when many miscellaneous variations and degenerations appear.
DIFFERENCES ARISING FROM PARTIAL ISOLATION (PORRISM).
Members of the same species are often more or less unlike in the different parts of their geographical range of distribution. Some of these differences will be found to have relations to differences of environment, but others will persist even when brought into the same conditions. These geographical diversities represent, no doubt, the results of partial isolation, and 'are of the same nature as the differences between species. If interbreeding were adequate, evolutionary progress would be kept uniform over the whole species, but if the organism is sedentary or lacking in facilities of dispersion local diversities may accumulate. 
|1Engler, A., 1904. Plants of the Northern Temperate Zone in their Transition to the High Mountains of Tropical Africa. Annals of Botany, 18: 539.
"I am convinced that in such cases the somewhat different climate is the cause of all or at least of a part of the modifications. Sometimes in connection with these new variations are also to be observed (cf. Cerastium caespitosum), which may become the beginning of other new forms. The constancy of such climatical adaptations may be a different one and often become fixed through a geological period. I may add that systematic studies have also convinced me that many of the xerophytes, and that a good deal (I do not say all) of the qualities of xerophytes, which are usually called adaptations for protection against a dry climate, are caused by the climate itself."
Individuals from neighboring localities may maintain the usual amount of similarity, but if specimens from remote parts of the geographic range of the species be compared they may prove notably different. If the climatic or other conditions of the two localities are unlike it is very natural to infer that this is the cause of the differences between their organic inhabitants.1
That this explanation may prove, in some cases, to be correct, does not justify us, however, in neglecting to perceive that the remote members of a species may have opportunities to accumulate diverse characteristics, much as though they belonged to two distinct species. The extent to which they can do this will depend upon the habits of the particular plant or animal. Sedentary species of animals or plants which have no means of securing wide dissemination of seeds or pollen, tend to manifest local divergencies. The cause of this is, apparently, that new characteristics appear in different parts of the range of the species more rapidly than they can be distributed through the whole interbreeding group. Thus the quail, or Virginia partridge, a nonmigratory bird extending from New England to Central America, shows a large number of appreciably different local varieties or subspecies, which might not exist if the bird were migatory and there were a more general intermingling of the members of the species. The differences which characterize such local subspecies may be quite the same, both in character and amount, as those which distinguish completely segregated species, but they are treated as subspecies because the distribution of the whole group still remains continuous, and provides a complete series of connecting links between the local forms which happen to be described as subspecies. 
The essential difference between a species and a subspecies does not lie, as commonly supposed, in the nature or amount of the differences as such. The practical question is whether two groups are actually separate in nature or are still connected. Subspecies may be more different than other completely segregated species. On the other hand, groups which are really segregated in nature and thus unable to interbreed, are by that fact on the road to the acquisition of specific differences. That they may not have become very different from each other does not prove that they are not good species or that it is undesirable to accord them recognition as such.
It does not follow, as some have supposed, that subspecies are always incipient species, or that there is any inherent force or tendency which will insure a subsequent separation into distinct species. The existence of these diverse local forms has not been shown to be any disadvantage to a species, and may, indeed, conduce to its greater vigor, since it tends, like heterism, to insure a certain amount of desirable diversity of descent.
If the habits of a species were to change in the direction of an increase of its power of dissemination and wide interbreeding, the local differences would tend to disappear, since new variations could then spread more rapidly throughout the whole group and render its evolutionary progress more uniform.
Porrism corresponds, inside the species, to many of the differences between species. It is true that when species of the same genus live in different environments and have different habits they usually have structural difference corresponding to their respective needs. Examples of such adaptations are frequent among the higher plants and animals, and their superficial similarity to artism inside the species has been the basis of the doctrine that evolution has been effected by environmental causes. The best corrective of this misapprehension is a study of one of the lower groups of plants and animals in which the same family, order or class has the same habits and the same place in the economy of nature. Many excellent examples will be found among the mosses, liverworts and algae among plants, and among the myriapoda and lower insects where the number and character of the diversity of the species is out of all imaginable  proportion with differences of conditions, habits or selective requirements. Hundreds of species, genera, families, and even orders, have been differentiated notwithstanding complete and long‑standing adjustment to the same kind of existence.
The multiplication of species under such circumstances has little reference to environment or to natural selection, and the characters by which the groups differ are not explainable on the basis of utility. The diplopod fauna of tropical Africa changes almost completely every thousand miles, but the tropical forest conditions under which a large proportion of the species live are, for their purposes, practically identical the world over. But with these wingless, slow‑moving creatures unable to bear exposure to daylight and dry atmosphere, the opportunities for segregation are greater than those for dissemination. The environment allows a wide freedom of choice, and evolution by means of useless changes has far outrun the natural selection of advantageous differences. As far as their external characters are concerned, these animals appear to have been quite as well adapted to their environment in the carboniferous age as they are to‑day, but they have not ceased to differentiate species, although preserving much more than in some groups the same general form. Indeed, the wealth of definite structural differences is, if anything, greater than among the higher insects, where the progress in adaptive structural changes would seem to have removed the necessity of accentuating the inconsequential differences which the diplopoda have utilized as means of evolutionary motion.
DIFFERENCES OF NEW VARIATIONS (NEISM).
Much of the heterism or normal individual diversity of the members of a species can be described as resulting from different combinations and proportions of what have been called the unit characters of the species. The interweaving of the lines of individual descent brings, as we know, an infinite diversity of form and features, and with these differences accentuated by environmental influences there is almost an infinity of possibilities of diversified characters in the same species. Nevertheless, the making of all possible permutations of the characters which  may exist in a species at any particular period would lead, after all, to no truly progressive change. Nothing is gained for evolutionary purposes by attempting to explain new characters merely as reversions or as new combinations.
Nor can such assumptions fully account for the facts, since it is often obvious that absolutely new and unprecedented evolutionary departures sometimes appear, which could not be accounted for by any combination of characters existing in the remaining members of the group. Such are the remarkable crests developed on a few of the anterior segments of East African millipedes of the family Oxydesmidae, specialized structures which are entirely without analogy in the remainder of the order Merocheta or, for that matter, of the entire class Diplopoda.
It would be altogether presumptuous, of course, to insist that any particular variation or mutation represented the very first appearance of its type in the history of the species. It is usual to ascribe variations to possible admixtures of blood at some point in the genealogy of the individual, near or remote. But these suggestions, even if justified for particular cases, should not be allowed to obscure the more fundamental consideration that the very idea of a progressive evolution implies the origination and development of new characters, both of form and of structure, and the opening of new environmental relations for the species.
Of the causes of new characters we are, as yet, in ignorance, but of their uses we need be in no doubt. New characters not only make evolution possible, but by true symbasic interbreeding they help to maintain the vitality or organic efficiency of the species. Neism reinforces heterism and contributes to evolutionary progress. New characters are not averaged away and obliterated by interbreeding, but are prepotent. They tend to spread throughout the species and to become more and more accentuated.
|1Cook, O. F., 1904. The Vegetative Vigor of Hybrids and Mutations. Proc. of Biological Society of Washington, 17: 83.|
That variation may bring an increase of the vegetative vigor or vital efficiency of the organism could not be more clearly shown than in the numerous instances where unusual bodily strength and hardiness accompany reproductive debility or even  complete sterility, as in the familiar instance of the mule.1 Many similar instances were observed in Guatemala. Coffee plantations which, owing to unfavorable conditions, were dead or dying, often showed occasional mutations which remained healthy and luxuriant. Through some strange internal difference they were able to carry on their vital functions with conspicuous success while all their normal neighbors had completely failed. If coffee were grown for the leaves like tea or for other vegetative parts, these mutations would furnish new types of great economic value, but of thousands of such variants which have come under the observation of planters not one has proved to be equal in fertility or normal seed production to the parent type, under favorable conditions.
PREPOTENCY OF NEW VARIATIONS.
If only a small proportion of the progeny showed the new character it might still gain a footing in the species, especially if favored by selection. Those who have relied on the mathematical doctrine of chance have felt it necessary to claim generous assistance from the principle of selection. Experiments with new variations seem all to agree, however, that among their own relatives, or under equal conditions of symbasis, they have not merely an equal chance of reproducing themselves, but that probabilities are distinctly in their favor. The variation is not resisted but welcomed. The majority does not set the fashion; it is the few who are able to make pleasing modifications of style. The new pattern may not be better or more beautiful than the old, but change is pleasing in itself and may secure a wide vogue for an ugly or uncomfortable garment. With organisms as with clothes the essence of beauty is fitness, as Socrates long ago pointed out. The changes which make a permanent contribution to evolutionary progress are those which fit best into the existing structure and increase its fitness to its surroundings. Our admiration for changes and likewise for fitness in nature and in art, may be an intellectual reflection of the evolutionary properties of organisms. 
DIFFERENCES OF ABERRANT HEREDITY (TERATISM).
There are many biological accidents, so to speak, as when in the laboratory, or perhaps in the surf of the sea beach, an egg of one of the simpler animals is shaken apart and develops into two organisms instead of one. In a similar manner, through some mistake of division, two‑headed monsters and other malformations occur. No less abnormal are many of the freaks which can be produced by unfavorable conditions of growth. Another series of abnormalities is caused by violations of the law of symbasis, that is, through inbreeding which eliminates heterism and normal diversity of descent.
Teratic characters which are the result of accidents of growth or environment are not inherited, except as they may give rise to a general weakness or debility of the organism. Teratic neisms, on the other hand, are readily heritable.
Teratisms, like accommodational variations, have received much study, especially from those who hoped to gain from organic derangements an insight into the nature of the agencies by which organic structures are built. The field of teratology affords many interesting and significant data, but the correct interpretation of them has been hindered, as in other departments of evolution, by the confusion of issues which are essentially distinct. There are at least as many kinds of teratisms as there are of normal differences, and probably more, and endless gradations of each kind. This is well illustrated by the phenomena of mutation which have received so large an amount of study in recent years. Mutations show all degrees of abnormality, and they grade imperceptibly into the differences of normal individual diversity (heterism) as well as into those of normal and prepotent new characters (neism).
ABNORMAL MUTATIVE DIVERSITY.
That species are not normally constant and stationary in their characters could not be better proved experimentally than by the many attempts of breeders of plants and animals to maintain constancy of characters in domesticated varieties. Selection conduces at first to such a constancy or uniformity among all [273 the members of the breed, those not conforming to the approved standard being ruthlessly weeded out. The type having been once established by this means, the variety remains for a period of years more or less uniform, generally very much more so than the members of wild species in nature. It is the experience of all history, however, that varieties decline after a time from their original excellence and have to be replaced by other, newer sorts, which by reason of their more recent origin have been subjected to shorter periods of inbreeding. The degeneration of the older variety may be indicated in a number of ways, such as a decline in fertility or weaker vegetative growth, or susceptibility to fungous and insect parasites, so that it usually disappears from cultivation or husbandry before the final stage of sterility and extinction is reached, though the tendency in this direction often becomes very obvious.
One of the symptoms of degeneration is the appearance of numbers of freaks, sports or mutations, as they are variously called. These variations of domesticated plants and animals are often interesting, and sometimes valuable on account of some special peculiarity, such as long hair, double flowers, albino color, etc. This is especially true among the plants cultivated for their flowers, where the never‑ending diversity of garden varieties is obtained by the preservation of the numerous mutations into which wild species commonly "break" after a period of domestication and inbreeding.
A general tendency among all such sorts is towards lessening of seed production, and finally complete sterility may ensue. The last is not a calamity in species which can be propagated by cuttings, and many of our cultivated species have reached this condition. With others, as for example, the "seedless" green‑house or forcing cucumbers, the extreme scarcity of seeds which renders the variety desirable is at the same time a serious obstacle to its cultivation.
On the strength of the older static, uniformitarian theory of life, some writers have insisted that mutations must be caused by environment, there being, in their opinion, nothing else to cause them. The diversity of the mutations could be explained, under this doctrine, only by environmental differences, such as  the variety of chemical compounds which might be found in the soil of the same seed‑bed. But no evidence of any constant relation between any particular chemical and any particular mutative character has been adduced. That any will be forthcoming may well be doubted, in view of the fact that the same or closely similar mutative characters often appear under very different conditions of soil and climate, and very diverse mutations under the same conditions.
The diversity of the mutations among themselves shows that it is not safe as yet to assert more than this general organic instability; detailed causes are not yet revealed. The necessity of this caution is rendered still more obvious by the behavior of neotopic mutations, those induced by changes of environmental conditions. If in a given environment a plant mutated only in one direction, we would still be far from knowing adequately that the environment caused the mutation, but even when we have reason to believe that a change of environment has induced mutation we are forbidden to go farther, because of the very great diversity of the mutations which the same change of environment or the same history of selective inbreeding can induce.
It has been shown in the discussion of neotopism that new conditions may conduce to the appearance of abruptly discontinuous mutative variations. The percentage of mutants is notably larger in some regions than in others, but even this does not compel us to believe that the conditions are the true cause of the mutations, in any detailed sense. They are rather to be thought of as merely the occasion of the change, by having brought the coffee, the cotton or the Capsicum the sooner to the point when it can no longer follow the hereditary road over which the individuals must travel to attain the ancestral type of adult form.
The mutative individuals are not to be thought of as the evolutionary pioneers of the species; they represent rather those who are falling out by the wayside. They may be classed together with normal new variations in the sense that they are outside of the specific norm or average, but they have a different position with reference to the evolutionary route of the species. They represent the criminals and cranks, but not the  leaders and reformers of the specific organization. For special agricultural purposes mutations are often extremely valuable, but when the desire is for the general improvement of the species or the race, the essentially degenerate nature of mutations cannot be left out of account.
The kinetic theory, if correct, shows that variations, to be of evolutionary value, must take place in the species, or in full contact with society, as it were, and not alone, or in disregard of the condition, interests, and evolutionary direction of the species at large.
Mutations are physiological phenomena, just as evolution itself is a physiological process; they will undoubtedly be found to have causes when we are able to appreciate them. They may be thought of as functional reactions from the restriction of normal heterism and diversity of descent. This abnormal condition of inadequate symbasis renders the organism unstable and it falls down, degenerates or mutates.
Inbreeding is to be studied as a condition of existence, and the manner in which the species reacts may be observed with the same propriety as any more purely environmental problem. Mutations may be abnormalities induced by abnormal conditions of descent, but the reaction which produces them need not be considered abnormal, since it is evidently the same tendency which contributes to the maintenance of the normal heterism.
Indeed, the mutations might restore the normal intraspecific diversity if interbreeding were permitted, as in nature. The very fact that mutations of plants so frequently tend toward dioecism might be accepted as another evidence of their value as a corrective of inbreeding and deficient heterism.
Coffee mutations are often largely or completely unisexual, or have greatly accentuated proterogyny or proterandry. A condition entirely analogous to a dioecious species could be obtained by the crossing of such staminate and pistillate trees. Nevertheless, Professor Dc Vries has described and named such a unisexual mutation as a new species, without regard to the taxonomic consequences of the application of this policy to sexually differentiated higher animals.
|1The oranges, lemons and pomellos afford, according to Mr. W. T. Swingle, many excellent examples of this parallelism of mutative variation.|
If similar results justify the predication of similar causes the  appearance of similar mutations under diverse conditions may be accepted as proof that they were induced by the common condition of inbreeding. Otherwise it would be necessary to suppose that different topic factors have produced like results, all of which shows the hopelessness of connecting mutations with environment. Mutations represent abnormally accentuated individual differences, and it seems not unlikely that most of them follow lines of variation already established within the species.1 It has been found in all the species thus far canvassed that a few mutative tendencies are much more frequently shown than the others.
Nevertheless, it is not safe to assume that the same mutation reappears even twice in identical forms. Whenever two similar mutations of coffee, cotton, or Capsicum have been brought together and compared they have always been found to be very distinctly different, even more so than the unmutated individuals of the uniform type from which they have arisen.