The Journal of Heredity 5: 155-158 (1914)
Organization through Interwoven Lines of Descent a Characteristic of Species—
Biology Not Advanced by Ignoring this Fundamental Condition—
Eugenics a Problem of Maintaining the Species, rather than of Separating Varieties.

Bureau of Plant Industry, U. S. Department of Agriculture, Washington. D. C.

That all the plants and animals are organized into species is a fundamental fact of biology. If the organization of species were confined to a few groups of plants or animals it would have been considered very remarkable and would have been made the subject of special investigation. But being universal and altogether familiar, the existence of species is taken entirely for granted in some branches of biology and as completely disregarded in others.

Darwin had deeper insight into the problems of evolution and heredity than many of his successors, because he saw that these problems relate to species; that is, to species in the sense of natural groups of interbreeding organisms. Taxonomic complications regarding species have been allowed to confuse the issues to such an extent that current writers on general biological problems leave the phenomenon of specific organization entirely out of account or even deny that species have any real existence. A single instance may serve to illustrate this tendency:

"We have devised a scheme whereby we regard animals as segregated into a series of groups—species, genera and so on—subordinated one to another. We arbitrarily separate these groups by sharp lines. While the scheme expresses, to some extent, our ideas concerning the past history of animals, the groups themselves have no real existence 'in nature,' as we say. There these sharp lines do not exist. The species or other group has no definite limits in space, no form, no integrity. It has no organization as a whole."

Plants and animals can be investigated in many ways without taking account of the organization of species. The interwoven lines of descent that bind the members of a species into a coherent organization may be separated, as in the breeding of varieties of our domesticated plants and animals, and endless experiments can be made with such varieties without considering their derivation from an ancestral specific organization. Many investigators prefer to study varieties rather than species because varieties have more uniform characters and more definite differences, so that the results of experiments are more readily reported in statistical form. When this preference is taken into account it is not surprising that theories of heredity should have been based upon facts drawn from the study of varieties, without considering the organization of the species.


As long as our interest is limited to the varieties that are brought into the garden or the breeding pen for experimental purposes, it may appear better policy to avoid reference to species, but in the study of eugenics this policy can hardly be maintained, for eugenics does not have the same purpose of developing specialized uniform varieties. The problem of eugenics is to learn how to improve the human species in other ways, without destroying individual diversity or the normal network of descent. Human progress, to be permanent and secure, must be made in normal evolutionary ways, instead of by any direct imitation of the method of producing our short‑lived varieties of domesticated plants and animals. Hence a recognition of the existence of species and an understanding of the organization of specific groups would seem to be necessary steps in the development of a science of eugenics. To overlook the specific organization of living matter is to leave out of account the fundamental and unique condition of organic existence and of evolutionary progress.

The obvious purpose of the study of evolution and heredity is to understand how the complex and highly specialized bodies of plants and animals have been built up, and how they are rebuilt in each generation. From an evolutionary viewpoint, it is apparent that the cellular organizations represented in the bodies of plants and animals have developed through the association and differentiation of cells, and that progress in cellular evolution is dependent upon the organization into groups of interbreeding individuals, the groups that are usually called species.

Among the lowest forms of life the individual organisms are merely single cells, and the cells are all of one kind, and all of these cells taken together constitute the species. Organization, in the sense of building up cellular structures, begins when the cells associate in colonies, instead of living as separate unicellular individuals. After the habit of living in colonies has become established, the species is thought of as made up of colony individuals, rather than of cell individuals. In other words, the specific organizations of single‑celled organisms furnished the materials for making colonies and building higher types of cellular structures.


1Cook, O. F., and Swingle, W. T. Evolution of Cellular Structures, Bulletin 81, Bureau of Plant Industry, U. S. Department of Agriculture, 1905.

The next stage of organization is reached when the members of a colony of cells are not all of the same kind, but become specialized in form and function. The building up of the higher types of cellular organization involves the association of vast numbers of cells. The specific organization and the cellular organization have made mutual progress. Special systems of reproduction have been developed, making it possible to build up these highly complex forms of organization. It has not been sufficient to have different kinds of cells to make up individual organisms. Diversification has been raised, as it were, to the second power, in that different kinds of polycellular individuals are represented in the specific organizations. Sexual and other forms of diversity among the members of species have developed in all of the higher groups, to accompany increasing specializations of cellular organization. 1

The dependence of the cellular organization upon the specific organization. continues and becomes more apparent as we ascend in the scale of the structural complexity, and is shown by the need of crossing of lines of descent through sexual reproduction. The species and the individual are morphological facts, and sexuality the physiological function that connects them. The species is a network of interwoven lines of descent and has as real an existence in nature as an individual animal or plant. The species produces the individual and the individual adds its share to the network of descent of the species. To think of the individual as producing itself without reference to the specific organization is like assuming the spontaneous generation of a complex cellular structure.

The true nature of a species as represented by a network of descent is not apparent if we consider only the few individuals that may be taken to represent the species at a particular moment of time. In order to see the network we must visualize the past and the future, in accordance with the facts of organic succession. Narrowing our view of a species to a short period of time is like focusing our attention on the cross‑section of a fabric. This enables us to examine the free ends of the individual threads, or to study an occasional knot, but we lose sight of the network as a whole.


2Cook, O. F. Physical Analogies of Biological Processes. The American Naturalist, 46: 493, 1912.

There is no structural organization without underlying specific organization. Organisms maintain their existence and make evolutionary progress only in species. It is the species, rather than the individual, that has truly biological existence. Evolution and heredity are collective, superorganic processes, hardly to be understood from the standpoint of the individual organism. Instead of disregarding species, students of general biological problems should consider the association of all plants and animals in species, in other words, the speciety of living matter, as one of the most significant and fundamental facts. That this condition or property of speciety is not analogous to any of the physical and chemical properties of unorganized matter, should give it a special interest for the biological investigator.2

The traditional analogy of the genealogical tree also gives a misleading idea of the nature of organic succession. The usual object of a genealogical tree is to show descent from a single ancestor, and unless consanguineous marriages occur, all the branching lines of descent remain separate and distinct. But this apparent distinctness is maintained only because each marriage is a grafting with a branch of a different family tree. If all the genealogical trees with their intergrafted branches could be represented, we would have a diagram corresponding to the network of descent of the species.

That the network of descent is lacking in definite form or external morphoIogical structure is not a reason for denying that the species has organization, for amoebae and many other living organisms lack definite form. This is true even of some highly specialized organs, such as the brain, whose functions are not at all dependent upon regularity of external form. Morphological specialization is, after all, only a secondary result of protoplasmic specialization. The conditions that determine protoplasmic efficiency must be recognized before we can hope to understand organization in the external, structural sense.

Some have thought to gain a better understanding of heredity by analyzing the diversified natural species into uniform pure lines, where the analogies of the physical sciences more nearly apply. Yet evolution is not an analytic process, but highly synthetic. The specific organization of interweaving lines of descent provides for the accumulation and combination of the desirable variations, those that render the species better adapted to its environment. To unravel the network of descent into separate lines may help us to understand some of the problems of heredity, but it does not represent the essential condition of normal, self‑perpetuating organic existence, or of evolutionary progress. None of the analytic experiments in the propagation of single lines of descent are permanently successful. If the crossing of the lines of descent is prevented, as by vegetative propagation or by artificial breeding, the individual structures become abnormal or weak and eventually cease altogether. Our vegetative varieties and pure bred strains are short‑lived. None of the higher types of plant or animal life are maintained without sexual reproduction in a specific network of descent.


To say that the power of organization is lodged in the individual germ cell is not the whole biological truth. The power resides rather in the specific organization, the protoplasmic network of the species, of which the individual germ is but a fragment. The vitality of the individual depends upon its relation to the organization of the species. The specific networks of descent, rather than the individual lines, have the power to develop and maintain the complex structures of the higher animals and plants. Organization in this sense of complex cellular structures is not a general property of protoplasm, for it is not manifested by the lowest forms of life, but is rather to be considered as an accomplishment of the higher forms, those that have developed complex specific organizations and correspondingly specialized sexual processes for maintaining the networks of descent of the species.

Why the interweaving of lines of descent in specific groups is necessary to maintain the power of cellular organization we do not know, but neither do we know why the overlapping of metal plates generates the electric current in a voltaic pile. The facts are established by repeated observation, and should not be disregarded in our attempts to understand related phenomena. Of course, these complexities of specific organization and sexuality are very unwelcome ideas to those who are about to solve the problems of evolution and heredity by simple experimental and statistical methods, but no truly biological investigation can disregard the fundamental fact that organisms exist in species.

That categories of classification of species are artificial is not an indication that the groups themselves have no real existence. As well might we say that continents and islands have no real existence because their shore‑lines are not definitely fixed and do not coincide with parallels of latitude and longitude which geographers use to determine locations. The object of biological classification is to find our way about in the endless diversities of organic nature, and for this purpose many conventional devices are employed, but these should not he allowed to conceal the more essential facts.


Students of classification are always seeking diagnostic differences between the various species and higher groups, for the task of discovering such characters and framing them into words is very difficult. Hence taxonomy makes only gradual progress, like other departments of science. Yet the formal nature of the concepts and categories of classification does not affect the concrete nature of the groups that are being classified. The categories of the biologist are artificial like those of the geographer, but not more so. Geographers do not agree in the grouping of the archipelagoes of the Pacific Ocean, but this is not considered a reason for denying that islands exist or that some islands are close together and others far apart. Species are biological islands, in a sea of nonexistence. The higher categories of classification, such as genera, families and orders, are more conceptual than species, in that they do not form coherent networks of lines of descent, but even this deficiency does not render them pure abstractions. They are to he consider rather as collective entities, that is, they are groups of species that stand in certain phylogenetic relations to each other, corresponding to the positional relations that determine geographic groups.

It is true that the boundaries of species often appear less definite in nature than in books, but the same is true of islands where the shore lines change with every tide, to say nothing of the more extensive and permanent changes by erosion of cliffs or elevation of beaches. Yet these incidental limitations need not destroy concrete ideas of islands as continuous bodies of land, nor of species as coherent groups of organisms. The members of each species are bound together by a network of lines interbreeding into a physiological unity, quite independent of morphological similarities or diversities inside the species. It is organization that constitutes the species, not the characters that may be ascribed to it.

The difficulties of classifying and discovering diagnostic characters for the several millions of species that exist on the earth's surface should not be allowed to confuse the minds of physiological and statistical workers, or to obscure the reality of specific organization. The specific structure or speciety of living matter is as truly a fact as any other biological phenomenon. That there should be so many species, and so difficult to classify, adds annoying complexities to biological investigation, but the difficulties are not removed by disregarding the existence of species. Sailors encounter similar difficulties in the navigation of archipelagoes, but do not find it safe to overlook any of the islands that lie in their courses.