Now the extreme importance of homology is manifest from the fact that the taxonomists of zoölogy and botany have found it to be the most satisfactory basis for a scientific classification of animals and plants. In both of these sciences, organisms are arranged in groups according as they possess in common certain points of resemblance whereby they may be referred to this, or that, general type. The resemblance is most complete between members of the same species, which do not differ from one another by any major difference, though they may exhibit certain minor differences justifying their subdivision into varieties or races. These morphological considerations, however, must, in the case of an organic species, be supplemented by the additional physiological criteria of perfect sexual compatibility and normal viability, as we have already had occasion to note in the previous chapter. When organisms, though distinguished from one another by some major difference, agree, notwithstanding, in the main elements of structure, the several species to which they belong are grouped under a common genus, and similarly genera are grouped into families. A relative major difference, such as a difference in the size of the teeth, suffices for the segregation of a new species, while an absolute difference, such as a difference in the number of teeth or the possession of an additional organ, suffices for the segregation of a new genus. In practice, however, the classifications of systematists are often very arbitrary, and we find the latter divided into two factions, the “lumpers” who wish to reduce the number of systematic groups and the “splitters” who have a passion for breaking up larger groups into smaller ones on the basis of tenuous differences. Above the families are the orders, and they, in turn, are assembled in still larger groups called classes, until finally we reach the phyla or branches, which are the supreme categories into which the plant and animal kingdoms are divided. As we ascend the scale of classification, the points of resemblance between the organisms classified are constantly decreasing in number, while the points of difference increase apace. Hence, whereas members of the same species have very much in common, members of the same phylum have very little in common, and members of different phyla show such structural disparity that further correlation on the basis of similarities becomes impossible (in the sense, at least, of a reliable and consistent scheme of classification), all efforts to relate the primary phyla to one another in a satisfactory manner having proved abortive.
Within the confines of each phylum, however, homology is the basic principle of classification. But the scientist is not content to note the bare fact of its existence. He seeks an explanation, he wishes to know the raison d’être of homology. Innumerable threads of similarity run through the woof of divergence, and the question arises: How can we account for the coëxistence of this woof of diversity with a warp of similarity? Certainly, if called upon to explain the similarity existent between members of one and the same species, even the man in the street would resort instinctively to the principle of inheritance and the assumption of common ancestry, exclaiming: “Like sire, like son!” It is a notorious fact that children resemble their parents, and since members of the same species are sexually compatible and perfectly interfertile, there is no difficulty whatever in the way of accepting the presumption of descent from common ancestral stock as a satisfactory solution of the problem of specific resemblance. Now, it is precisely this selfsame principle of heredity which the Transformist invokes to account for generic, no less than for specific, similarity. In fact, he presses it further still, and professes to see therein the explanation of the resemblances observed between members of the different families, orders, and classes, which systematists group under a common phylum. This, of course, amounts to a bold extension of the principle of inheritance far beyond the barriers of interspecific sterility to remote applications that exceed all possibility of experimental verification. Transformists answer this difficulty, however, by contending that the period, during which the human race has existed, has been, geologically speaking, all too brief, and characterized by environmental conditions much too uniform, to afford us a favorable opportunity for ascertaining the extreme limits to which the genetic process may possibly extend; and, even apart from this consideration, they say, racial development (phylogeny) may be, like embryological development (ontogeny) an irreversible process, in which case no recurrence whatever of its past phenomena are to be expected in our times.
Be that as it may, the evolutionist interprets the resemblances of homology as surviving vestiges of an ancient ancestral type, which have managed to persist in the descendants notwithstanding the transformations wrought in the latter by the process of progressive divergence. Moreover, just as the existence of a common ancestor is inferred from the fact of resemblance, so the relative position in time of the common ancestor is inferred from the degree of resemblance. The common ancestor of forms closely allied is assumed to have been proximate, that of forms but distantly resembling each other is thought to have been remote. Thus the common ancestor of species grouped under the same genus is supposed to have been less remote than the common ancestor of all the genera grouped under one family. The same reasoning is applied, mutatis mutandis, to the ancestry of families, orders and classes.
The logic of such inferences may be questioned, but there is no blinking the fact that, in practice, the genetic explanation of homology is assumed by scientists to be the only reasonable one possible. In fact, so strong is their confidence in the necessity of admitting a solution of this kind, that they do not hesitate to make it part and parcel of the definition of homology itself. For instance, on page 130 of Woodruff’s “Foundations of Biology” (1922), we are informed that homology signifies “a fundamental similarity of structure based on descent from a common antecedent form.” The Yale professor, however, has been outdone in this respect by Professor Calkins of Columbia, who discards the anatomical definition altogether and substitutes, in lieu thereof, its evolutionary interpretation. “When organs have the same ancestry,” he says, “that is, when they come from some common part of an ancestral type, they are said to be homologous.” (“Biology,” p. 165.) In short, F. A. Bather is using a consecrated formula culled from the modern biological creed when he says: “The old form of diagnosis was per genus et differentiam. The new form is per proavum et modificationem.” (Science, Sept. 17, 1920, p. 259.)
A moment’s reflection, however, will make it clear that, in thus confounding the definition proper with its theoretical interpretation, the modern biologist is guilty of a logical atrocity. Homology, after all, is a simple anatomical fact, which can be quite adequately defined in terms of observation; nor is the definition improved in the least by having its factual elements diluted with explanatory theory. On the contrary, the definition is decidedly weakened by such redundancy. And as for those who insist on defining homology in terms of atavistic assumption instead of structural affinity, their procedure is tantamount to defining the clear by means of the obscure, an actual effect by means of a possible cause. Moreover, this attempt to load the dice in favor of Transformism by tampering with the definition of homology ends by defeating its own purpose. For, if homology is to serve as a legitimate argument for evolution, then obviously evolution must not be included in its definition; otherwise, the conclusion is anticipated in the premise, the question is begged, and the argument itself rendered a vicious circle.
Having formed a sufficiently clear conception of homology as a static fact, we are now in a position to consider the problem of its causality with reference to the solution proposed by evolutionists. Transmutation, they tell us, results from the interaction of a twofold process, namely, the conservative and similifying process called inheritance, and progressive and diversifying process known as variation. Inheritance by transmitting the ancestral likeness tends to bring about uniformity. Variation by diverting old currents into new channels adjust organisms to new situations and brings about modification. Homology, therefore, is the effect of inheritance, while adaptedness or modification is the product of variation.
As here used, the term inheritance denotes something more than a mere recurrence of parental characters in the offspring. It signifies a process of genuine transmission from generation to generation. Strictly speaking, it is not the characters, such as coloration, shape, size, chemical composition, structural type, and functional specificity, that are “inherited,” but rather the hereditary factors or chromosomal genes, which are actually transmitted, and of which the characters are but an external expression or manifestation. Hence, it is scarcely accurate to speak of “inherited,” as distinguished from “acquired,” characters. As a matter of fact, all somatic characters are joint products of the interaction of germinal and environmental factors. Consequently, the external character would be affected no less by a change in the environmental factors than by a change in the germinal factors. In a word, somatic characters are not the exclusive expression of the genetic factors, but are equally dependent upon environmental influence, and hence it is only to the extent that these characters are indicative of the specific constitution of the germ plasm that we may speak of them as “inherited,” remembering that what is really transmitted to the offspring is a complex of genes or germinal factors, and not the characters themselves. The sense is, therefore, that “inherited” characters are manifestative of what is contained in the germ plasm, whereas “acquired” characters have no specific germinal basis, but are a resultant of the interaction between the somatic cells and the environment. In modern terminology, as we have seen, the aggregate of germinal factors transmitted in the process of reproduction is called the genotype, while the aggregate of somatic characters which manifest these germinal factors externally is spoken of as the phenotype. Only the genotype is transmitted, the phenotype being the subsequent product of the interplay of genetic factors and environmental stimuli, dependent upon, and expressive of, both.
Variation, therefore, may be based upon a change in the germ plasm, or in the environment, or in both. If it rests exclusively upon an extraordinary change in the environmental conditions, the resulting modification is non-inheritable, and will disappear so soon as the exceptional environmental stimulus that evoked it is withdrawn. If, on the contrary, it is based upon a germinal change, it will manifest itself, even under ordinary, i.e. unchanged or uniform environmental influence. In this case, the modification is inheritable in the sense that it is the specific effect of a transmissible germinal factor, which has undergone alteration.
As we have seen in the foregoing chapter, there are three kinds of germinal change which result in “inheritable” modifications. The first is called factorial mutation, and is initiated by an alteration occurring in one or more of the chromosomal genes. The second is called chromosomal mutation, and is caused by duplication (or reduction) of the chromosomes. The third may be termed recombination, one type of which results from the crossover or exchange of genes between pairing chromosomes (“pseudomutation”), the other from random assortment in accordance with the Mendelian law of the independence of allelomorphic pairs. This so-called “random assortment of the chromosomes” is the result of the shuffling and free deals of the chromosomal cards of heredity which take place twice in the life-cycle of organisms: viz. first, in the process of gametic reduction (meiosis); second, in the chance meeting of variously-constituted sperms and eggs in fertilization. A mischance of the first of these “free deals” is bewailed in the following snatch from a parody belonging to the Woods Hole anthology.
“Oh chromosomes, my chromosomes,