USDA Bureau of Plant Industry, Bulletin No. 81, 1905.
EVOLUTION OF CELLULAR STRUCTURES
O. F. Cook and Walter T. Swingle.

INTRODUCTION.

The value of a new point of view lies in the fact that it permits new relations to be perceived. By means of the kinetic theory of evolution it has become possible to understand that organic development has been carried forward through gradual improvement of the methods of descent rather than by environmental causes. Instead of there being a law of heredity which tends to keep the individuals of a species uniform, or exactly alike, the tendency, especially among higher plants and animals, is to maintain, inside the species, a diversity of form and structure, most conspicuously manifested in the phenomenon of sex.

This intraspecific diversity is neither accidental nor incidental, but of great physiological and evolutionary importance. The interweaving of distinct lines of descent is necessary to sustain the strength and vital efficiency of the individual organisms, and to continue the evolutionary progress by which the species adapts itself to changing environments or enters new ones. Interbreeding is as indispensable for the species as for the individual, or even more so, for seedless plants continue their individual existences after the coherence of the specific group has been lost.

Normal and long-sustained evolutionary progress is not accomplished on single or narrow lines of descent, but is possible only for large companies of interbreeding individuals; that is to say, for species. It is thus no mere accident, but a fundamental necessity, which brings about the association of organic individuals into species and determines what might be called the specific constitution of living matter. Species are sexual phenomena; they have come where they are only through symbasis; that is, as groups of interbreeding individuals, traveling together along the evolutionary pathway.

This interpretation of familiar biological facts is supplemented and confirmed by the study of the processes of cell conjugation, which are the means of symbasic interbreeding. Among simple organisms conjugation is a periodical incident in the multiplication of equal and independent cells. Higher stages of organization were reached by the production within the same species of many kinds of cells and the building of these into large colonies or compound individuals. There was, however, a very early limit to the structures which could be built of the primitive, simple type of cells, as illustrated by the filament of the lower alga, the vegetative mycelium of the fungus, and the thallus of the liverwort. The plant series would have culminated, apparently, with the leafy axis of the moss if the basis of organization had not been changed from the primary or simple type of cells to the double or sexual type.

   a) The terminology followed in this paper presupposes for convenience the existence of the cellular type of organization common to most animals and plants. The conclusions here reached apply with equal force, however, to organisms such as the Infusoria among the protozoa, the Siphonocladiaceae among the algae, and the Saprolegniales and Mucorales among the fungi, in all of which groups considerable structural differentiation is attained without any division of the organism into cells. Such forms as Caulerpa and Acetabularia among the Siphonocladiaceae reach a considerable size and even show a well-marked differentiation into the analogue of stem and leaf, rhizome and root, without the enormously expanded thallus being divided up into cells at all, although very numerous nuclei arise by subdivision and are scattered throughout the cytoplasm. These nuclei could be double, in which case such plants would be directly homologous to the double-celled organisms described in the following pages.

In undifferentiated unicellular or equal-celled (isocytic) organisms the successive generations of cells may be thought of as joined into a network by an occasional conjugation. The cells at the knots of the network are, as we know, double, being formed from the association of two nuclei and the accompanying protoplasms. They are often strikingly different from the remainder of the cellular fabric of descent, and have been given special names, such as oospore, zygospore, and resting spore. In the find or lowest category of sexual organisms only one cell in each generation is double; there is only one large bead at each node of the genealogical network. (See Pl. 1.) A second type of organic structure was initiated when an organism attained the art of forming two or more of these double cells by division.a It is of such double cells that all the higher plants and animals are built. The new type of organization was not merely supplementary to the old; it was a new biological invention, giving rise to a new category of vitality, which not only outstripped the old type of structural organization, but even caused it to be abandoned and eliminated as a worse than useless impediment.

Organisms which were farthest ahead on the primitive basis have fallen far behind since the new course of development was opened. In such groups as the liverworts, mosses, and ferns the diversity of the two types of organic structure is strikingly obvious, and has received extensive study for years past under the name of "alternation of generations." Ample homologies have been found in the highest groups of plants to show that the so-called alteration of generations was everywhere in ancestral condition, and that all have followed essentially the same history in having abandoned the simple type of cell for the double as the basis of structural development.

   a) Strasburger, Edward. Annals of Botany, 1894, 8: 281-316.

That these converging data pointed to something of fundamental evolutionary significance has been confidently believed since the publication a decade ago of Strasburger's epoch-making essay entitled "The Periodic Reduction of the Number of the Chromosomes in the Life-History of Living Organisms,"a but a new evolutionary standpoint was required before the larger import of the facts could be perceived. The reduction of chromosomes is indeed a striking and unique phenomenon in the life history of organisms, and it naturally became the focus of interest in the rapidly developing science of cytology. A new point of view was the more necessary, however, because of an unfortunate choice of terms which has undoubtedly tended to prevent the perception of the true relations of the facts, as it now interferes with a correct description of them. We refer to the characterization of the higher, double-celled, spore-bearing "generation" as "asexual." Appreciating the primitive character of such structures as the prothallus in ferns, Strasburger asserted that a new "asexual generation" had been intercalated into the life history of organisms. It is now perceived that for cytological purposes this is not the whole truth, and that for evolutionary purposes it is not true at all. The new "generation" was not merely intercalated into the life history of the organism; it was intercalated into the sexual process. It is, therefore, not asexual, but sexual, and in a higher degree than the so-called sexual generation. The latter hears, it is true, the cells which conjugate, but the former is produced during the actual process of conjugation. Organic perfection has been attained, not through the development of an "asexual generation," but by the lengthening out of the sexual process itself; not by abandoning or avoiding sexuality, but directly by means of it.

Among the lower plants the single cell formed by conjugation accomplishes in a brief space of time all the cytological processes which in the higher plants come between fertilization and chromosome reduction. Sexual fusion is immediate and complete, and takes place during a brief period of interruption of the growth and subdivision of vegetative cells. If the vegetative fern prothallus is to be termed sexual because it produces antheridia and archegonia, the sporophyte is sexual to the second degree, for it is built of conjugating cells, containing, until synapsis and the subsequent reducing divisions, a double number of chromosomes, the parental chromatin elements being still unfused. However important chromosome reduction may be, it is, after all, only a corollary or sequel of the doubling conjugation. It was not the reducing division, but the long postponement of the reduction division, which permitted higher types of organisms to be developed by means of double, sexual cells.

A special evolutionary significance was ascribed to the chromosome reduction because cytology was approached from the standpoint of the somatic tissues of the higher plants and animals. This current interpretation reverses, however, the historical course of events. The reducing division was not an expedient incidental to the adoption of a process of sexual reproduction by organisms previously sexless. It was not the reduction to fewer chromosomes, but the retention of the double number, that constituted the important step in sexual reproduction and made possible the evolution of complex higher organisms. It is, therefore, not the reducing division, but the doubling conjugation, which should constitute the datum point or base line for tracing cytological homologies.

THE ELIMINATION OF THE SIMPLE-CELLED PHASE.

Chromosome reduction brought about by synapsis, or the fusion of the chromatin elements, followed by two special nuclear divisions, is not, historically speaking, the beginning of the sexual process, but the end of it. Chromosome reduction stands in no special causal relation to the subsequent conjugation. The number of cell generations formed between synapsis and conjugation differs greatly in the various natural groups, and merely shows how far the organism still adheres to its old simple-celled life history. Fecundation and synapsis, as the beginning and the end of the sexual process, would seem to be directly comparable in all organisms which have developed a double-celled sexual phase.

From the physiological standpoint, it may be an advantage to dispense with the simple-celled phase and thus shorten the period between the chromosome reduction which marks the end of one conjugation and the cell fusion which begins another. Synapsis relieves organic fatigue by means of new nuclear configurations, and has been thought of as & .stimulant of vital activity or energy of growth, the benefit of which can be secured for the new double-celled structure by very prompt conjugation, as occurs in all the higher plants and animals. This consideration would help to explain the organic inferiority of such a group as the ferns, which, although they have developed a double-celled phase, continue to waste the energy derived from synapsis on a worse than useless simple-celled structure.

   a) Farmer, J. B., and Moore, J. E. S. On the Maiotic Phase (Reduction Divisions) in Animals and Plants, in Quarterly Journal of Microscopical Science, n. s., No. 192 (vol. 48. No. 4), Feb., 1905, pp. 489-557, pl. 34.

   b) To recognize, however, as Farmer and Moore do, these two cell generations as a distinct "maiotic phase" in the life history of Metaphyta and Metazoa does not seem warranted, since chromosome reduction is apparently a mechanical necessity resulting from sexual conjugation and is consequently brought about in a practically identical manner in all symbasic organisms, from the lowest to time highest. Maiosis is rather a connecting link at the node in the network of descent than a distinct phase subject to expansion or contraction as organisms mount in the scale of evolutionary progress. On the other hand it is clear that the two peculiar cell generations occurring during maiosis can not properly be classed with time double-celled phase that usually precedes or with the simple-celled phase that usually follows, but constitute a transition stage marking the end of one generation and the beginning of another.

In all animals above protozoa this reduction of the simple-celled phase has gone so far as to result in its complete elimination, for the two peculiar, nuclear divisions which occur in rapid succession immediately after synapsis constitute an essential part of chromatin reduction. That these phenomena noted are indissociably connected stages in the process of chromosome reduction has been emphasized recently by Farmer and Moore,a who propose the convenient term maiosis to include synapsis and the subsequent heterotype and homotype nuclear divisions.b

ALTERNATION OF STRUCTURAL TYPES.

   b) Bower, F. O. A Theory of the Strobilus in Archegoniate Plants. Annals of Botany, 8: 343-365. 1894.

"Alternation of generations" is an expression borrowed from zoology; its application to the archegoniate plants has introduced endless complexities, and can be justified, after all, only by false analogies. Alternation of generations was discovered by Chamisso in a species of Salpa, a marine animal belonging to the Tunicates: but here, as well as in the traditional zoological example of the Aphides, or plant lice, the phenomena have entirely different evolutionary significance from the so-called antithetic alternation of generations in the archegoniate. Generations or individual life cycles of Salpa and of plant lice, which were originally alike, have become different, so that now parthenogenetic generations alternate with sexual generations. To make the archegoniate plants a parallel instance, it would be necessary to assume that what is now called the sporophyte was originally another thallus, or something that corresponded to one, which later on became modified into the sporophytic "generation." To state the case in this way may seem quite superfluous, since nobody has made such a suggestion. Strasburger and others have repeatedly declared that the so-called asexual generation had been intercalated—that is, added anew—and not substituted for something else. This, however, makes it only the more obvious that the sporophyte is a generation only in a very loose and inaccurate sense, and not because it corresponds to or takes the place of any other generation. The simple fact is that, instead of forming merely one oospore as the result of fertilization, the archegoniates have come to form a whole sporophyte or double-celled structure by the multiplication and progressive sterilization of potentially sporogenous tissue, as Bower has shown.b

   a) It is clear that the expansion of the fertilized egg could occur in siphonaceous algae and fungi without any cross walls forming between the nuclei as they arise by subdivision. The mature thallus of an Acetabularia is obviously the enormously expanded syngamete and may or may not contain double nuclei. On the other hand, the Infusoria may be found to consist of one double cell, the successive cell generations not adhering to form a tissue.

Bower's generalization is, however, only a half truth, since the sterilization, or, better, the arrest of spore formation of some of the cells, is conditioned on the possibility of continued subdivision and growth of the fertilized egg, and this can occur only when there is a definite postponement of some stage of sexual fusion, for if the final stage is once reached and the chromatin fuses, no further growth is possible, and a new generation is inaugurated automatically. When sexual fusion is immediate and complete, i.e., when nuclear fusion follows close on cell conjugation and is in turn at once succeeded by chromatic fusion, no development of the oospore can occur; it simply breaks up into four spores. Such was once the fate of the eggs of all organisms, and such is still their fate in the lower plants. All development of the fertilized egg other than a simple splitting into four spores is due to an arrest of the process of sexual fusion which permits its expansion into a mass of double cells, such as constitute the bodies of higher animals and plants.a It is, however, clear that the effect of such an arrest in the process of sexual conjugation and consequent intercalation of a double-celled phase in the life history of the organism is to lengthen the life cycle; it lessens the number of generations instead of making more of them.

   b) Hofmeister, W., Vergleichende Untersuchungen der Keimung, Entfaltung und Fruchtbildung hôherer Kryptogamen und der Samenbildung der Coniferen. Leipzig. 1851.

   c) This fact is obscured, but not negated, by the splitting up during chromosome reduction of the egg and sperm mother cells of animals into four gametes, which are simple cells, but which are no longer capable of further development unless they conjugate. As previously noted (p. 13, footnote a), these two cell generations occurring during chromosome reduction constitute a transition stage between the old and the new generations and can not properly be classed with the simple-celled phase.

Notwithstanding half a century of endeavor, botanists and zoologists have not yet found in the higher animals any definite homologue of the so-called antithetic alternation of generations discovered by Hofmeisterb in the archegoniate plants. The whole idea of alternating generations must, however, be abandoned and emphasis placed instead on the expansion of the oospore or fecundated egg into a double-celled phase that comes to occupy a larger and larger part of the life cycle as organisms mount higher in the scale of evolutionary progress. It then becomes evident that in higher animals (Metazoa) the expansion of this phase has gone so far that the simple-celled stage has been completely suppressed, and in consequence their life history is as free from alternating phases as that of the lower plants, though for a very different reason. The lower groups show no expansion of the fertilized egg. The higher animals consist of nothing else.c

The occurrence of alternating phases in the life history of an organism indicates that it is in an unstable evolutionary condition, since it has not yet attained the most effective type of organization. The persistence of a clearly two-phased condition in the vascular cryptogams and of a reduced alternation of phases, even in the highest algae and flowering plants, is a proof of the extreme slowness of the evolutionary progress of the plant kingdom. Animals seem to have passed through the diphase period of their existence before the dawn of geologic history, and appear in the oldest fossil-bearing strata, not only as completely double-celled organisms but highly differentiated ones at that. Not only are there no traces of the two-phased progenitors which must have gone before the lowest known fossil organisms, but up to now zoologists have not realized the need of postulating such forms at all, end have been content to derive the higher animals from merely simpler but always completely double-celled ancestors, which, of course, are not primitive. It seems not improbable that the completely double-celled condition has been reached independently by different groups of higher animals, just as it has been approximated, though not attained, by the Fucaceae and the phanerogams among plants. The animal kingdom dose not contain, so for as is now known, a single species that shows alternating phases in its life history; it has no counterparts of all that wealth of forms which in the plant kingdom bridge the interval from the protophytes to the flowering plants.

That there are two unicellular stages in the life history of an organism should not be allowed to introduce any confusing technicalities. For genealogical purposes the spore is quite as much the descendant of the antherozoid and the egg cell as it would be if the other tissues of the sporophyte had not been intercalated. From chromosome reduction to chromosome reduction, from spore to spore, or from egg to egg is one generation, and not two. The prothallus is no more mysterious than any other piece of ancient history. The ferns were originally liverworts, the capsules of which had the good fortune to get roots into the ground and keep on growing, but they have not yet learned to dispense with their first vain attempt at building a structure on a simple-celled basis.

SEXUALITY A MECHANISM OF EVOLUTION.

   a) "The modifications introduced into palingenesis by kenogenesis are vitiations, strange, meaningless additions to the original, true course of evolution."—Haeckel, Evolution of Man, vol. 11, p. 460, note 9.

Stress has also been laid upon this supposed alternation of "sexual" and "asexual" generations in the belief that a clew was here to be gained regarding the nature of sex and of attendant "mechanisms of heredity." But since only one generation is really involved instead of two, and since the phase of existence which has been termed asexual is in reality the more strongly sexual, it is not surprising that these expectations have not been realized. Sexuality facilitates interbreeding and makes it the more effective by distributing new variations throughout the species; it is, in short, a mechanism of diversification and of evolution, a fundamental and universal fact which stands squarely in the way of the alleged law of heredity under which organisms would breed true and be exactly alike. This notion of a uniform and unchanging heredity,a or of any natural tendency to such a condition of organic stagnation, can well be relegated to the limbo of hypotheses which have not proved useful. Heredity is not a mechanism or a force; it is merely another name for the property of organic continuity or succession. There is no more heredity in an organism at one stage of its life history than at another.

Sexual and other diversities inside specific lines are not useless morphological complexities or mere failures in the execution of a fundamental plan of complete uniformity. Diversity, interbreeding, and evolution are physiological factors of the highest importance in maintaining vital efficiency.

Morphologically speaking, sexuality is a specialization of the internal diversity of the species, and among plants, at least, it has been attained independently in a large number of unrelated natural groups. There are grades of sexual differentiation just as there are of organic structures. Moss plants and fern prothalli may be sexually differentiated and the differentiation may occur farther hack in the spore itself, or even in the sporophyte or double-celled phase, as in the flowering plants and the higher animals. Thus in the same species there may be two sexualities, one in the simple-celled stage and another in the double, and these may have no homology or causal connection, except as they both serve the same purpose of promoting more efficient symbasis. Indeed, the sexuality of the highest types of organization is not merely double, but threefold; the individual has sex, as a whole; the double cells of which the body is composed are a part of a sexual process, and the simple cells which it produces for the initiation of a new generation are sexually differentiated.

TWO TYPES OF DOUBLE-CELLED STRUCTURES.

That organisms are everywhere associated in species is not because of some undiscovered principle or mechanism of heredity; it is simply because the interweaving of the lines of individual descent is being maintained, without which the specific association would be dissolved into indefinite radial divergence and degeneration, as among the varieties of bananas and other plants long propagated from cuttings. Many explanations have been conjectured for the supposed absence of sexual reproduction among the higher groups of fungi. From the standpoint of a symbasic evolution, however, it becomes evident that the existence of true, coherent species among these fungi is a sufficient evidence of interbreeding, and hence of sexuality. There is in many groups a deficiency of specialized sexual organs, but these are rendered unnecessary by abundant opportunities for direct conjugation among the mycelial filaments.

That the cells of the more complex reproductive tissues of the higher fungi are known to have two nuclei, while in the younger mycelium there is only one, might also have been accepted as proving that conjugation had taken place. This does not mean, of course, that cross-fertilization is indispensable for spore production among the fungi, but their habits of growth certainly give many opportunities for conjugations between mycelia of different descent, by which the existence of compact and well-defined species can be maintained, although the peculiar structure of fungous tissues permits extreme variability of the size and external form of the fruiting bodies.

In structural complexity, size, longevity, and other measures of organic efficiency the binucleate fungi have an intermediate position in the plant series. Their wide distribution and extensive differentiation into species, families, and orders are evidences of ample opportunities in time and environment, so that it is not unfair to explain their evolutionary limitations by reference to their peculiar type of organic structure.

   a) The etymology of these terms will be obvious to all students of biology, plasma and karyon being the familiar Greek renderings of protoplasm and nucleus, respectively. The other element, αψις, signifies a binding or tying together and also a mesh or network, a meaning especially appropriate in view of the reticular structure of living matter.
   The series might be completed more logically by using the distinctive word mitapsis as a substitute for synapsis, which in its etymology is scarcely more than a Greek equivalent for the general term conjugation. Mitapsis is derived from μιτος, a thread, and alludes to the threadlike condition assumed by the chromatin during the process of chromatic fusion.

Sexual reproduction is accomplished through conjugation or fusion of cells, a process which may be divided into three stages: (1) Plasmapsis, the fusion of the cytoplasm or unspecialized protoplasm; (2) karyapsis,a the fusion of the nuclei or nuclear protoplasm; (3) synapsis, the fusion of the chromatin. The binucleate cells of the fungi may be said to have passed the stage of plasmapsis, but karyapsis, or true fecundation, like that of the higher plants and animals, has not taken place.

For the form of sexuality which produces the binucleate cell structures of the higher fungi the name apaylogamy is proposed, in allusion to the fact that the two nuclei have not yet associated. The higher stage, where the nuclei fuse but the chromosomes remain apart, may be called paragamy, which implies that the union is still incomplete, but that a more intimate relation has been established. These two double-celled conditions may be further contrasted with haplogamy, the primitive method of undeferred combination of the sexual cells, nuclei, and chromatin.

To the "asexual generation," which is not asexual and not a generation, the term paragamic phase may be applied among the higher plants and animals, the tissues of which are composed of cells with a double number of chromosomes. The binucleate structures of the fungi may be referred to as the apaylogamic phase. The so-called "sexual generation" may be called the haplogamic phase in both cases. These new terms might not be necessary if words were used for descriptive purposes only, but in the present instance they have general implications too important to be disregarded.

Haplogamic structures are built between synapsis and plasmapsis, apaylogamic between plasmapsis and karyapsis, paragamic between karyapsis and synapsis. Between the three critical points of cytological activity there are three intervals, in which the organism can pause to gain additional size or numbers by vegetative division of cells. The relations between the cell structures and the nuclear processes are illustrated by the accompanying diagram (fig. 1).

No organisms have, however, structures built in all the three phases. The relative importance of each phase in the life histories of the different natural groups can also be illustrated by simple diagrams, as shown in figure 2.

The relative importance of the different phases in the life history of the various groups of organisms can be represented in another way, as is shown on Plate I. The diagrams on this plate show in addition a network of descent in its simplest form, composed of successive generations linked together at the first stage of conjugation. The generations themselves are seen to be composed of alternating simple and double celled phases in organisms of intermediate evolutionary rank, and finally the double-celled phase is shown to be an expansion of the fertilized egg, which constitutes an increasingly large part of the life history as organisms mount higher in the scale of evolutionary progress.

   a) As noted before, some organisms, such as Caulerpa and Acetabularia, show a considerable degree of evolutionary progress, and have not as a matter of fact attained the cellular type of organization at all; they may, however, be found to have double nuclei and to be very striking examples of the expansion of the fertilized egg.

It is thus easy to understand why the two types of double-celled structures have very unequal possibilities of organization. Two nuclei are evidently better than one, but their association is too slight, apparently, to gain much of the vital stimulus consequent upon the more effective method of conjugation followed by the higher plants, where the chromosomes of two fused nuclei lie in juxtaposition in the new nucleus. The higher organisms have not merely double cells, but, what seems to be vastly more important, compound nuclei, a more advanced and energized stage of the sexual process, which enables them to maintain for exceedingly long periods of time the power of growth and subdivision.a

The intercalation of the double-celled structure does not change the order of nuclear events in cross-fertilization, but it may be said to change fundamentally their chronological and physiological relations. The true historical sequence of conjugation is plasmapsis, karyapsis, and synapsis, but the apparent and practical sequence in the higher plants and animals becomes synapsis, plasmapsis, and karyapsis, the synapsis which ends one conjugation being followed closely by the plasmapsis which begins another. The suspension of nuclear changes for vegetative growth no longer occurs between synapsis and plasmapsis, but between karyapsis and synapsis, the double-celled, paragamic structure being built, as already stated, on a new and highly sexual plane, that is, out of cells in a state of prolonged sexual union.

If, as may be supposed, the benefit of synapsis lies in the making of new associations of the ancestral chromatin elements, it is obvious that the bringing of two such newly energized nuclei together would produce a condition which, for want of other words, might be called a multiple vital tension. The double-celled type of structure involves, therefore, not merely a transfer of emphasis to a new part of the lifecycle, but a new and improved sexual process, which raises the biological equation to a higher power. From this standpoint it is obvious that the morphological diversities of sex have a fundamentally important and truly physiological function in building up and maintaining the efficiency of the complex organization of the higher plants and animals. It is as illogical to ascribe the internal diversity of species to external environmental causes as to arbitrary mechanisms of heredity.

The extent to which the static concept of a normally unchanging heredity has obscured evolutionary thought and investigation could not be better shown, perhaps, than by the fact that, notwithstanding the great multiplicity of terms which have been proposed for all the imagined kinds of variations, no name has been suggested for this normal and necessary intraspecific diversity. The deficiency maybe made good by the use of the word heterism for the whole group of phenomena, ranging from mere individual diversity to the highest specializations of heterism, exemplified by sexes, castes, and polymorphic conditions. It is true that the members of a species look alike when compared with those of other species, and there may be no harm in ascribing this likeness to heredity, but there is nothing to show that this heredity of general resemblance has anything to do with evolution except as an incidental result. Evolution does not take place between species, but inside of them; it is not an interspecific phenomenon, but intraspecific. Its principal factors are heterism and symbasis, not heredity and environment, as believed by the selectionists, nor heredity and segregation, as supposed by the mutationists.

HEREDITY IN RETICULATE DESCENT.

The greater efficiency of the double nuclei is, however, only one more evidence of the importance of sex as a means of diversity and of bringing diverse protoplasms together. The nuclear network of chromatin which controls the activities of the cell corresponds to the network of descent through which the cell has come into existence. Symbasis, or diversity of descent with normal interbreeding, is the foundation of the strength and vitality of the organism, because it increases the efficiency of the nuclei of the component cells.

Inbreeding or defective fertilization, on the other hand, would cause nuclear deterioration, as so strikingly shown by Maupas in the so-called senile degeneration of ciliate Infusoria induced by keeping them too long without cross-fertilization. This phenomenon is, indeed, closely parallel to senile degeneration, but there is, nevertheless, an important difference. In true senile degeneration the vigor of the cells is declining because of the absence or long postponement of conjugation. In monobasic degeneration, conjugation may take place, but is not effective because of insufficient diversity of descent. Monobasis is the antithesis of symbasis; it means descent without cross-fertilization, on single or very narrow lines. The inevitable result is degeneration, with a rapidity proportional to the closeness of the inbreeding and the complexity of the organisms.

This intimate relation between organic descent and organic structure enables us to understand the phenomena of organic succession without resorting to abstract principles or to hypothetical mechanisms of heredity. The network of descent is, as it were, a map showing the alternative mutes of the developing organism, and permitting normally any combination of ancestral characters, as may well result from the endlessly varying arrangements into which the ancestral chromatin elements may fall at the time of synapsis or chromatic fusion. Twins developed from the same ovum would have the same arrangement of chromatin, which accords with their close similarity of form, but otherwise there is unlimited diversity, even among the simultaneous offspring of the same parents. It would seem, therefore, that instead of a mechanism of heredity inside the reproductive cells there is an automatic device for insuring diversity. The higher the organization the more complex the descent, and the greater the variety of nuclear configurations and the resulting individual diversity.

   a) Cook, O. F. Natural Selection in Kinetic
Evolution, Science, n. s., 19: 549. 1904.

Nevertheless, inheritance is not governed merely by chance, nor limited even to the infinity of nuclear networks to be made by the combinations possible among the ancestral chromatin elements. With the greater vitality of interbred organisms is associated also a stronger heredity or prepotency of the wild or more broadly symbasic types when such are crossed with inbred domesticated varieties. New variations, too, appear to have the same effect as diversity of descent, in lending greater vigor and prepotency. Even mutations, or degenerative variations induced by inbreeding, are prepotent on their own plane of symbasis—that is, when crossed only with their own inbred relatives—though they are promptly obliterated or "swamped" when brought into contact with the broadly symbasic wild type, the prepotency of the diverse descent being far greater than that attaching to the inbred variation. It is the prepotency of variations which renders evolution truly kinetic; for the methods of organic descent are such as to bring about a spontaneous change of type. The environment often influences the direction of this vital motion, but is in no proper sense an actuating cause.a

Cells are the units of organization, but species, as groups of interbreeding individuals, are the units of evolution. The causes of evolution are not revealed by hypothetical subdivisions of cells into character units or determinate elements, but by ascertaining the methods of descent through which interbreeding maintains organic strength and evolutionary progress. Cells divide themselves, as we know, into other cells, and species into other species, but it is only as cells and as species that their vital, organic, evolutionary activities are accomplished. Individuals vary and mutate, but only species evolve. To classify the various stages and functions of organisms tinder general and abstract terms may be desirable, but for evolutionary purposes it is the network of descent which represents the concrete, significant fact, and it is this which can be resolved, if necessary, into its component lines, polygons, or nodes, to furnish units for the calculation of quantitative effects of inheritance, as in Galton's Law of Ancestral Resemblances and Filial Regression, under conditions of normal symbasic descent, or in Mendel's Laws of Disjunction, in hybrids of abnormally inbred varieties.

The recognition of the double character of the cells of the higher plants and animals permits many other phenomena of inheritance to be understood, though it seems to take us farther than ever from the hope of a merely mechanical explanation of the nature of heredity itself. If conjugation were concluded immediately, the well-known phenomena of sterile hybrids would be impossible, the sterility which puts an end to their existence being due, as now known, to the failure to perform synapsis or chromatin fusion. On the other hand, it may be that crosses between narrowly inbred varieties sometimes have the power of passing by the period of synapsis without a true fusion of the parental chromatin, perhaps in a manner corresponding to that in which Thalictrum produces seeds parthenogenetically, by avoiding chromosome reduction. The germ-cells might have a preponderance of chromatin from one parent or the other, or might even be quite unmixed, as claimed for Mendelian hybrids. It is obvious, however, that to explain Mendelism in this manner is to admit the essential abnormality of the phenomenon.

SUMMARY.

It has been held self-evident that there can be nothing in evolution except heredity and environment, and it was a simple deduction from such an aphorism that differences must all be due to environment, since "heredity would, if nothing interfered, keep the descendants perfectly true to the physical characters of their progenitors." Such heredity, however, is a pure figment of the scientific imagination; it is a hypothesis which lends us no aid in understanding the facts of organic succession. A stereotyped heredity could make nothing new; the interbreeding of diverse individuals and the prepotency of new variations are the constructive factors, not heredity and environment.

Symbasis is the method, interbreeding the means, and sexuality the mechanism whereby organic evolution has been accomplished; these are the concrete and efficient causes of the vital motion of species. The association of organisms into species of similar individuals is not brought shout by a predetermining hereditary mechanism, but by symbasic interbreeding. The highest organization has not been attained in "asexual generations," but in structures completely and essentially sexual, built wholly of conjugating cells. There has been no evolution away from sexuality. Long-continued violations of the law of symbasis bring only degeneration.

This interpretation of evolutionary facts opens the way to an adequate physiological explanation of the significance of sex, and affords also a working theory of the chief cytological complications that have arisen as a consequence of sex—complications that have hitherto rendered obscure the nature of the cell-bodies of higher animals and plants.

The external diversity of organic nature and the internal diversity of cells and of reproductive processes take on new and unexpected significance. Both are shown to be consequences of sexual specialization, without which no evolutionary advance beyond simple-cell colonies has been possible. More than this, gradations in the perfections of the higher double-celled structure are correlated with definite stages of evolutionary progress, so that from the structure of an organism its kind of sexuality can be deduced. Evolution becomes, in the new view, a physiological rather than a morphological process, since the methods of descent affect the quality and efficiency of the organism even more promptly and fundamentally than they do its external form.

The circles (O), eights (8), and thetas (Θ) represent in each case the nucleus or nuclei belonging to a cell, and the succession of cell generations is shown by a string of nuclei either simple, in pairs (apaylogamic double cells), or fused (paragamic double cells). The half circles () and quadrants () represent the two cell generations formed during chromosome reduction. The brackets [ ] represent a cell at the period or periods when the organism is reduced to a unicellular condition. All the signs for nuclei could be supposed to be inclosed by a cell wall, which has been omitted for the sake of clearness. For the same reason only the cell lineage leading directly up to the formation of the gametes has been shown, and no account has been taken of the enormous multiplication of cells which occurs not only to build up the bodies of animals and plants but also to form many gametes. Only a few of the numerous cell generations which make up the organisms in question are shown.

EXPLANATION OF SIGNS.

Plasmapsis—fusion of the cytoplasm, or unspecialized protoplasm.
Karyapsis—fusion of the nuclei, or nuclear protoplasm.
Synapsis—fusion of the chromatin elements.
Heterotypic and homotypic divisions following synapsis.
Nuclei of haplogamic phase—structures composed of simple cells having nuclei and chromatin elements completely fused.
Nuclei of apaylogamic phase—structures composed of double cells, each having two unfused nuclei.
Nuclei of paragamic phase—structures composed of double cells having single nuclei containing unfused chromosomes.
Cell, at periods where the organism is reduced to a single cell.
The expanded egg.

EXPLANATION OF FIGURES.

FIG. 1.—Lower organism, such as Sphaeroplea, having only simple-celled (haplogamic) tissues. The fertilized egg undergoes no development beyond merely splitting up into four spores when it germinates.

FIG. 2—Higher fungus, such as Agaricus or Puccinia, showing alternation of simple-celled and double-celled phases, the latter apaylogamic, i.e., with two unfused parental nuclei in each cell. The fertilized egg has expanded into a mass of apaylogamic tissue.

FIG. 3.—Moss, showing alternation of a long simple-celled and a short double-celled phase, the latter paragamic, i.e., with parental nuclei fused but with their chromosomes still distinct and unfused. The fertilized egg has expanded slightly into a mass of paragamic tissue.

FIG. 4.—Fern, showing alternating phases as in moss (figure 3), but with a short simple-celled phase and a long double-celled phase, the paragamic phase having developed at the expense of the haplogamic. The fertilized egg has expanded very much into a mass of paragamic tissue.

FIG. 5.—Flowering plant (phanerogam), showing alternation of a very short simple-celled phase with a very long double-celled phase, the paragamic phase having developed greatly at the expense of the haplogamic. The egg mother-cell develops only one cell (macrospore). The fertilized egg expands into a large mass of paragamic tissue in which the greatly reduced haplogamic phase develops in a semiparasitic manner, it having no free existence.

FIG. 6.—Higher animal, having only double-celled tissues, the haplogamic phase having been completely suppressed by the greatly expanded paragamic phase. The egg mother-cell develops only one egg. The fertilized egg has expanded into a large mass of tissue.

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