CHAPTER II. Homology and Its Evolutionary Interpretation

HOMOLOGY AND ITS EVOLUTIONARY INTERPRETATION

The recent revival of interest in the problem of evolution seems to have called forth two very opposite expressions of opinion from those who profess to represent Catholic thought on this subject. M. Henri de Dorlodot, in his “Le Darwinisme,” appears in the rôle of an ardent admirer of Darwin and an enthusiastic advocate of the doctrine of Transformism. The contrary attitude is adopted by Mr. Alfred McCann, whose “God—or Gorilla” is bitterly antagonistic not only to Darwinism but to any form whatever of the theory of Transformism. Both of these works possess merits which it would be unjust to overlook. Dorlodot deserves credit for having shown conclusively that there is absolutely nothing in the Scriptures, or in Patristic tradition, or in Catholic theology, or in the philosophy of the Schools, which conflicts with our acceptance of organic evolution as an hypothesis explanatory of certain biological facts. In like manner, it must be acknowledged that, even after a liberal discount has been made in penalty of its bias and scientific inaccuracy, Mr. McCann’s book still contains a formidable residue of serious objections, which the friends of evolution will probably find it more convenient to sidestep than to answer.

Unfortunately, however, neither of these writers maintains that balanced mental poise which one likes to see in the defenders of Catholic truth. Dorlodot seems too profoundly impressed with the desirability of occupying a popular position to do impartial justice to the problem at issue, and his anxiety to keep in step with the majority blinds him apparently to the flaws of that “Darwinism” which he praises. Had he been content with a simple demarcation of negative limits, there would be no ground for complaint. But, when he goes so far as to bestow unmerited praise upon the author of the mechanistic “Origin of Species” and the materialistic “Descent of Man”; when, by confounding Darwinism with evolution, he consents to that historical injustice which allows Darwin to play Jacob to Lamarck’s Esau, and which leaves the original genius of Mendel in obscurity while it accords the limelight of fame to the unoriginal expounder of a borrowed conception; when, by means of the sophistry of anachronism, he speciously endeavors to bring the speculations of an Augustine or an Aquinas into alignment with those of the ex-divinity student of Cambridge; when he assumes that Fixism is so evidently wrong that its claims are unworthy of consideration, whereas Transformism is so evidently right that we can dispense with the formality of examining its credentials; when, in a word, he expresses himself not merely in the sense, but in the very stereotyped cant phrases of a dead philosophy, we realize, with regret, that his conclusions are based, not on any reasoned analysis of the evidence, but solely upon the dogmatism of scientific orthodoxy, that his thought is cast in antiquated molds, and that for him, apparently, the sixty-five years of discovery and disillusionment, which have intervened since the publication of the “Origin of Species,” have passed in vain.

But, if Dorlodot represents the extreme of uncritical approval, Mr. McCann represents the opposite, and no less reprehensible, extreme of biased antagonism, that is neither fair in method nor conciliatory in tone. Instead of adhering to the time-honored practice of Catholic controversialists, which is rather to overstate than to understate the argument of an adversary, Mr. McCann tends, at times, to minimize, in his restatement, the force of an opponent’s reasoning. He frequently belittles with mere flippant sneer, and is only too ready to question the good faith of those who do not share his convictions. Thus, when McCann ridicules Wells and accuses him of pure romancing, because the latter speaks of certain hairy “wild women” of the Caves, he himself seems to be ignorant of the fact that a palæolithic etching has been found representing a woman so covered with hair that she had no need of other apparel (the bas-relief from Laugerie-Basse carved on reindeer palm—cf. Smithson. Inst. Rpt. for 1909, p. 540 and Plate 2).

Mr. McCann may object, with truth, that this is far from being a proof that the primitive representatives of the human race were hairy individuals, but the fact suffices, at least, to acquit Mr. Wells of the charge of unscrupulous invention. Hence, while we have no wish to excuse the lamentable lack of scientific conscientiousness so manifestly apparent in the writings of popularizers of evolution, like Wells, Osborn, and Haeckel, nevertheless common justice, not to speak of charity, constrains us to presume that, occasionally at least, their departures from the norm of objective fact were due to ordinary human fallibility or to the mental blindness induced by preconceptions, rather than to any deliberate intent to deceive. And we feel ourselves impelled to make this allowance for unconscious inaccuracy all the more readily that we are confronted with the necessity of extending the selfsame indulgence to Mr. McCann himself. Thus we find that the seventh illustration in “God—or Gorilla” (opposite p. 56) bears the legend: “Skeletons of man and chimpanzee compared,” when, in point of fact, the ape skeleton in question is not that of a chimpanzee (Troglodytes niger) at all, but of an Orang-utan (Simia satyrus), as the reader may verify for himself by consulting Plate VI of the English version of Wasmann’s “Modern Biology,” where the identical illustration appears above its proper title: “Skeleton of an adult Orang-utan.” Since the error is repeated in the index of illustrations and in the legend of the third illustration of the appendix, it is impossible, in this instance, to shift the responsibility from Mr. McCann to the printer. In any case, it is sincerely to be hoped that this, and several other infelicitous errors will be rectified in the next edition of “God—or Gorilla.”

In the next chapter we shall have occasion to refer again to Dorlodot’s book. For the present, however, his work need not concern us, while in that of Mr. McCann we single out but one point as germane to our subject, namely, the latter’s inadequate rebuttal of the evolutionary argument from homology. The futility of his method, which consists in matching insignificant differences against preponderant resemblances, and in exclaiming with ironic incredulity: “Note extraordinary resemblances!” becomes painfully evident, so soon as proper presentation enables us to appreciate the true force of the argument he is striving to refute. Functionally the foot of a Troglodyte ape may be a “hand,” but structurally it is the homologue of the human foot, and not of the human hand; nor is this homology effectually disposed of by stressing the dissimilarity of the hallux, whilst one remains discreetly reticent concerning the similarity of the calcaneum. For two reasons, therefore, the irrelevance of Mr. McCann’s reply is of special interest here: (1) because it illustrates concretely the danger of rendering a refutation inconsequential and inept by failing to plumb the full depth of the difficulty one is seeking to solve; (2) because it shows that it is vain to attempt to remove man’s body from the scope of this argument by citing the inconsiderable structural differences which distinguish him from the ape, so that, unless the argument from homology proves upon closer scrutiny to be inherently inconclusive, its applicability to the human body is a foregone conclusion, and implies with irresistible logic the common ancestry of men and apes.

Such are the reflections suggested by the meager measure of justice which Mr. McCann accords to the strongest zoölogical evidence in favor of evolution, and they contain in germ a feasible program for the present chapter, which, accordingly, will address itself: first, to the task of ascertaining the true significance of homology in the abstract as well as the full extent of its application in the concrete; second, to that of determining with critical precision its intrinsic value as an argument for the theory of transmutation.

Homology is a technical term used by the systematists of botany, zoölogy and comparative anatomy to signify basic structural similarity as distinguished from superficial functional similarity, the latter being termed analogy. Organisms are said to exemplify the phenomenon of homology when, beneath a certain amount of external diversity, they possess in common a group of correlated internal resemblances of such a nature that the organisms possessing them appear to be constructed upon the same fundamental plan. In cases of this kind, the basic similarity is frequently masked by a veneer of unlikeness, and it is only below this shallow surface of divergence that we find evidences of the identical structure or common type.

Thus organs of different animals are said to be homologous when they are composed of like parts arranged in similar relation to one another. Homologous organs correspond bone for bone and tissue for tissue, so that each component of the one finds its respective counterpart in the other. The organs in question may be functionally specialized and externally differentiated for quite different purposes, but the superficial diversity serves only to emphasize, by contrast, the underlying identity of structure which persists intact beneath it. Thus, for example, the wing of a pigeon, the flipper of a whale, the foreleg of a cat, and the arm of a man are organs differing widely in function as well as outward appearance, but they are called homologous, none the less, because they all exhibit the same basic plan, being composed of similar bones similarly disposed with respect to one another.

Organs, on the other hand, are called analogous which, though fundamentally unlike in structure, are, nevertheless, superficially modified and specialized for one and the same function. The wing of a bird and the wing of an insect furnish a trite instance of such analogy. Functionally they subserve the same purpose, but structurally they bear no relation to each other. In like manner, though both are devoted to the same function, there exists between the leg of a man and the leg of a spider a fundamental disparity in structure.

At times, specialization for the selfsame function involves the emergence of a similar modification or uniform structural adaptation from a substrate of basic dissimilarity. In these instances of parallel modifications appearing on the surface of divergent types, we have something more than mere functional resemblance. Structure is likewise involved, albeit superficially, in the modification which brings about this external uniformity. In such cases, analogy is spoken of as convergence, a phenomenon of which the mole and the mole-cricket constitute a typical example. The burrowing legs of the insect are, so far as outward appearance goes, the exact replica on a smaller scale of those of the mole, though, fundamentally, their structure is quite unlike, the mole being built on the endoskeletal plan of the vertebrates, whereas the mole-cricket is constructed on the exoskeletal plan characteristic of the arthropods. Speaking of the first pair of legs of the mole-cricket, Thomas Hunt Morgan says: “By their use the mole-cricket makes a burrow near the surface of the ground, similar to, but of course much smaller than, that made by the mole. In both of these cases the adaptation is the more obvious, because, while the leg of the mole is formed on the same general plan as that of other vertebrates, and the leg of the mole-cricket has the same fundamental structure as that of other insects, yet in both cases the details of structure and the general proportions have been so altered that the leg is fitted for entirely different purposes from those to which the legs of other vertebrates and other insects are put.” (Quoted by Dwight in “Thoughts of a Catholic Anatomist,” p. 235.) In the analogies of convergence, therefore, we have the exact converse of the phenomenon so often encountered in connection with homology. The latter exhibits a contrast between basic identity and superficial diversity, the former a contrast between superficial convergence and fundamental divergence.

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, How sad is my condition! My grandsire’s gift for writing well Has gone to some lost polar cell And so I write this doggerel, I cannot do much better.”

These kinds of variation, however, in so far as they fall within the range of actual observation, are confined within the limits of the organic species. Intra-specific variation, however, will not suffice. To account for the adaptive modifications superimposed upon underlying structural identity, Transformism is obliged to assume the possibility of trans-specific variation. Yet in none of the foregoing processes of variation do we find a valid factual basis for this assumption.

Factorial mutation, for instance, waiving its failure to produce naturally-viable forms, or to meet the physiological sterility test of a new species, admits of interpretation as a change of loss due to the “dropping out” of a gene from the germinal complex. Bateson’s conception of evolution as a process consisting in the gradual loss of inhibitive genes, whose elimination releases suppressed potentialities, seems rather incredible. Many will be inclined to see in Castle’s facetious epigram a reductio ad absurdum of Bateson’s suggestion; for, according to the latter’s view, as the Harvard professor remarks, we should have to regard man as a simplified amœba. Certainly, it seems nothing short of a contradiction to ascribe the progressive complication of the phenotype to a simplification of the genotype by loss.

On the other hand, not only is there no experimental evidence of a germinal change by positive acquisition, that is, by the addition of genes, but it is hard to conceive how such a change could come about. “At first,” admits Bateson, “it may seem rank absurdity to suppose that the primordial form or forms of protoplasm could have contained complexity enough to produce the divers types of life.” “But,” he asks, “is it easier to imagine that these powers could have been conveyed by extrinsic addition? Of what nature could these additions be? Additions of material can not surely be in question. We are told that salts of iron in the soil may turn a pink hydrangea blue. The iron cannot be passed on to the next generation. How can iron multiply itself? The power to assimilate iron is all that can be transmitted. A disease-producing organism like the pebrine of silkworms can in a very few cases be passed on through the germ cells. But it does not become part of the invaded host, and we can not conceive it taking part in the geometrically ordered processes of segregation. These illustrations may seem too gross; but what refinement will meet the requirements of the problem, that the thing introduced must be, as the living organism itself is, capable of multiplication and of subordinating itself in a definite system of segregation?” (Heredity, Smithson. Inst. Rpt. for 1915, p. 373.)

Nor can we agree with Prof. T. H. Morgan’s contention that the foregoing difficulty of Bateson has been solved by the discovery of the chromosomal mutation. All unbalanced chromosomal mutants are subnormal in their viability and vitality, not to speak of their marked sterility. Haploidy represents a regressive, rather than a progressive, step. The triploid mutant is sterile. The tetraploid race of Daturas is inferior in fertility to the normal diploid plant. The origin of balanced tetraploidy from diploidy must be presumed, since it has never been observed. Moreover, tetraploidy represents only quantitative, and not qualitative, progress. The increased mass of the nucleus produces an enlargement of the cytoplasm, the result of which is giantism. This effect, however, is not specific; for giant and normal races possessing each the same number of chromosomes are known to exist in nature. Hence giantism may be due to other causes besides chromosomal duplication. The only effect of this doubling is a reinforcement and intensification of the former effect of the genetic factors, their specificity remaining unchanged. Double doses are substituted for single doses of the factors, but nothing really new is added. Morgan himself recognizes that this mere repetition of identical genes is insufficient, and that their multiplication must be qualitative as well as numerical, to answer the specifications of a progressive step in evolution. Hence he suggests that the chromosomal mutation is subsequently supplemented by appropriate factorial mutation. Once this supposition is made, however, all the objections we have mentioned in connection with factorial mutation (e.g. the subnormality of its products, its intra-specific nature, etc.) return to plague the speculator, and, in addition to these, he is confronted with the new difficulty of explaining how the redundance of duplicate genes can be removed and replaced by coördinate differentiation in their respective specificities. Now we have no factual evidence whatever of such a solidaric redifferentiation of the germinal factors, that would modify harmoniously the composition and rôle of each and every gene in the factorial complex. Nor is there any possibility whatever of accounting for this telic superregulation of the germinal regulators upon a purely mechanistic basis. How can the ultimate chemical determinants of heredity be thus redetermined? Consequently, although there is gametic incompatibility between diploid races and the tetraploid races, which are said to have arisen from the former, we are not, nevertheless, warranted, by what has been experimentally verified, in regarding tetraploid races as new species, or as progressive steps in the process of organic evolution.

To conclude, therefore, we have experimental verification of the efficacy of the similifying process said to have been at work in evolution, namely, inheritance. The same, however, cannot be said of the correlative diversifying process of trans-specific variation, which is said to have superficially modified old structures into new species. The latter process, accordingly, is but a pure postulate of science known to us only through the effect hypothetically assigned to it, namely, the adaptive modification.

The adaptation, however, of which there is question here is not to be confounded with the “acquired adaptation” of Lamarckian fame; for, unlike the latter, it is an inheritable modification rooted in the germ plasm. Adaptations of this sort do, indeed, adjust the organism to its external environment, but they are innate and not acquired. Hence they are often spoken of as preadaptations; for they precede, in a sense, the organism’s contact with the environing element to which they adjust it. They may possibly, it is true, have been acquired in the distant past, but they have now a specific germinal foundation, and no one was ever privileged to witness their initial production de novo. The whale, for example, though fundamentally a warm-blooded mammal, is superficially a fish, by reason of such a preadaptation to its marine environment. Preadaptation is of common occurrence, especially among parasites, symbiotes, commensals, and inquilines. Wasmann cites innumerable instances of beetles and flies so profoundly modified, in accommodation to their mode of life as guests in termite nests, that the systematist hesitates to classify them under any of the accepted orders of insects. Here the adaptive modification so disturbs the underlying homology as to make of these creatures taxonomical ambiguities. In the case of Termitomyia, he tells us, “the whole development of the individual has been so modified that it resembles that of a viviparous mammal rather than that of a fly.” (“The Problem of Evolution,” pp. 14, 15.)

Such modifications, however, amount to major, and not merely minor, differences. We are not dealing, therefore, with varietal distinctions here, but with specific, generic, and even ordinal differences. With reference to the phenomenon of adaptive modification, three things, consequently, are worthy of note: (1) it has the semblance of being adventitious to the underlying structural uniformity; (2) it is of such magnitude that it cannot be ascribed to variation within the species; (3) it has been appropriated by the hereditary process, in the sense that it is now an “inherited” character based on the transmission of specific germinal factors.

It may be remarked, in passing, that experimental genetics and mutation furnish no clue to the origin of adaptive characters. The Lamarckian idea alone gives promise in this direction. Orthogenesis leaves unsolved the mystery of preadaptation; yet only orthogenetic systems of evolution can be constructed on the basis of genetical facts. “Mutations and Mendelism,” says Kellogg, “may explain the origin of new species in some measure, but they do not explain adaptation in the slightest degree.” (Atlantic Monthly, April, 1924, pp. 488, 489.) We have seen in the previous chapter that they are impotent to explain in any measure the origin of new species.

Now it is claimed that for the occurrence of this kind of modification in conjunction with homology only one rational explanation is possible, and that explanation is evolution. If this contention be a sound one, and Dorlodot, who claims certitude for the evolutionary solution, insists that it is such, then, in the name of sheer logical consistency, but one course lies open to us. We cannot stop at Wasmann’s comma, we must press on to the very end of the evolutionary sentence and sing with the choristers of Woods Hole:

Rev. Erich Wasmann, S. J., accepts the evolutionary inference from homology as regards plants and animals. When it comes to man, however, he attempts to draw the line, and argues painstakingly against the assumption of a bestial origin of the human body.

“It’s a long way from Amphioxus, It’s a long way to us; It’s a long way from Amphioxus, To the meanest human cuss. Good-bye fins and gill slits; Welcome skin and hair. It’s a long, long way from Amphioxus, But we came from there.”

In this predicament it will not do, as we shall see presently, to adopt Mr. McCann’s expedient of balancing anatomical differences against anatomical resemblances. To do so is to court certain and ignominious defeat. We must, therefore, examine the argument dispassionately. If it be solid, we must accept it and give it general application. If it be unsound, we must detect its flaws and expose them. Intellectual honesty allows us no alternative!

Moreover, in weighing the argument from organic homology we must not lose sight of the two important considerations previously stressed: (1) that the inference of common ancestry in the case of homologous forms is based, not upon this or that particular likeness, but upon an entire group of coördinated resemblances; (2) that the resemblances involved are not exterior similarities, but deep-seated structural uniformities perfectly compatible with diversities of a superficial and functional character. “Nothing,” says Dr. W. W. Keen, “could be more unlike externally than the flipper of a whale and the arm of a man. Yet you find in the flipper the shoulderblade, humerus, radius, ulna, and a hand with the bones of four fingers masked in a mitten of skin.” (Science, June 9, 1922, p. 605.)

In fact, the resemblances may, in certain instances, be so deeply submerged that they no longer appear in the adult organism at all and are only in evidence during a transitory phase of the embryological process. In such cases, the embryo or larva exhibits, at a particular stage, traces of a uniformity completely obliterated from the adult form. In short, though frequently presented as a distinct argument, embryological similarity, together with all else of value that can still be salvaged from the wreck of the Müller-Haeckel Law of Embryonic Recapitulation, is, at bottom, identical with the general evolutionary argument from homology. In the latter argument we are directed to look beneath the modified surface of the adult organism for surviving vestiges of the ancestral type. In the former, we are bidden to go deeper still, to the extent, that is, of descending into the very embryological process itself, in order to discover lingering traces of the ancestral likeness, which, though now utterly deleted from the transformed adult, are yet partially persistent in certain embryonic phases.

In sectioning a larval specimen of the fly-like termite-guest known as Termitoxenia Heimi, Father Wasmann came across a typical exemplification of this embryological atavism. In the adult insect, a pair of oar-like appendages replace the wings characteristic of the Diptera (flies). These appendages are organs of exudation, which elaborate a secretion whereof the termites are very fond, and thereby render their possessors welcome guests in the nests of their hosts. The appendages, therefore, though now undoubtedly inherited characters, are the specific means by which these inquilines are adapted to their peculiar environment and mode of life among the termites. Moreover, the organs in question not only differ from wings functionally, but, in the adult, they bear no structural resemblance whatever to the wings of flies. Nevertheless, on examining his sections of the above-mentioned specimen, Wasmann found a developmental stage of brief duration during which wing veins appeared in the posterior branches of the embryonic appendages. Now, assuming that Wasmann’s technique was faultless, his specimen normal, and his interpretation correct, it is rather difficult to avoid his conclusion that we have here, in this transitory larval phase, the last surviving vestige of ancestral wings now wholly obliterated from the adult type, that, consequently, this wingless termite guest is genetically related to the winged Diptera, and that we must see in the appendages aboriginal wings diverted from their primitive function and respecialized for the quite different purpose of serving as organs of exudation, (cf. “Modern Biology,” p. 385.) Indeed, phenomena of this kind seem to admit of no other explanation than the atavistic one. It should be remembered, however, that Wasmann does not appear to have verified the observation in more than one specimen, and that a larger number of representative specimens would have to be accurately sectioned, strained, examined and interpreted, before any reliable conclusion could be drawn.

This transitory lymphatic, or tracheal venation appearing in the appendages at the stenogastric stage may not have the particular significance that Father Wasmann assigns. Such venation, even if vestigial and aborted, need not necessarily be a vestige of former wing venation. To demonstrate the validity of the atavistic interpretation, all other possible interpretations would have to be definitively excluded.

Such, in its most general aspect, is the atavistic solution of the problem presented by the homology of types. In it, similarity and diversity are harmoniously reconciled, in the sense that they affect, respectively, different structural, or different developmental, levels. It is futile, therefore, to look for contradictions where they do not exist. In a word, the attempt to create opposition between a group of basic and correlated uniformities, on the one hand, and some particular external difference, on the other, is not only abortive, but absolutely irrelevant as well. The reason is obvious. Only when likeness is associated with unlikeness is it an argument for Transmutation. Likeness alone would demonstrate Immutability by indicating a process of pure inheritance as distinguished from the process of variation. Hence evolutionists do not merely concede the coëxistence of diversity with similarity, they gladly welcome this fact as vitally necessary to their contention.

Now it is precisely this point which Mr. McCann, like many other critics of evolution, fails utterly to apprehend. Consequently, his efforts to extricate the human foot from the toils of simian homology are entirely unavailing. To offset the force of the argument in question, it is by no means sufficient, as he apparently imagines, to point to the fact that, unlike the hallux of the ape, the great toe in man is non-opposable (cf. “God—or Gorilla,” pp. 183, 184, and legends under cuts opposite pp. 184 and 318). The evolutionist will reply at once that the non-opposability of man’s great toe is correlated with the specialization of the human foot for progression only, as distinguished from prehension; while, in the ape, whose foot has retained both the progressive and the prehensile function, the hallux is naturally opposable in adaptation to the animal’s arboreal habits. He will then call attention to the undeniable fact that, despite these adaptational differences, the bones in the foot of a Troglodyte ape are, bone for bone, the counterparts of the bones in the human foot and not of those in the human hand. He will readily concede, that, so far as function and adaptedness go, this simian foot is a “hand,” but he will not fail to point out that it is, at the same time, a heeled hand equipped with a calcaneum, a talus, a navicular, a cuboid, and all other structural elements requisite to ally it to the human foot and distinguish it from the human hand. In fact, Mr. McCann’s own photographs of the gorilla skeleton show these features quite distinctly, though he himself, for some reason or other, fails to speak of them. It is to be feared, however, that his adversaries may not take a charitable view of his reticence concerning the simian heel, but may be inclined to characterize his silence as “discreet,” all the more so, that he himself has uncomplimentarily credited them with similar discretions in their treatment of unmanageable facts. In short, Mr. McCann’s case against homology resembles the Homeric hero, Achilles, in being vulnerable at the “heel.” At all events, the homology itself is an undeniable fact, and it is vain to tilt against this fact in the name of adaptational adjustments like “opposability” and “non-opposability.” Since, therefore, our author has failed to prove that this feature is too radical to be classed as an adaptive modification, our only hope of exempting the human skeleton from the application of the argument in question is to show that argument itself is inconsequential.

Mr. McCann’s predicament resembles that of the unlucky disputant, who having allowed a questionable major to pass unchallenged, strives to retrieve his mistake by picking flaws in a flawless minor. As Dwight has well said of the human body, “it differs in degree only from that of apes and monkeys,” and “if we compare the individual bones with those of apes we cannot fail to see the correspondence.” (“Thoughts of a Catholic Anatomist,” p. 149.) In short, there exists no valid anatomical consideration whatever to justify us in subtracting the human frame from the extension of the general conclusion deduced from homology. Whosoever, therefore, sees in the homology of organic forms conclusive evidence of descent from a common ancestor, cannot, without grave inconsistency, reject the doctrine of the bestial origin of man. He may still, it is true, exclude the human mind or soul from the evolutionary account of origins, but, if homology is, in any sense, a sound argument for common descent, the evolutionary origin of the human body is a foregone conclusion, and none of the anatomical “differences in degree” will avail to spare us the humiliation of sharing with the ape a common family-tree. It remains for us, then, to reëxamine the argument critically for the purpose of determining as precisely as possible its adequacy as a genuine demonstration.

To begin with, it must be frankly acknowledged that here the theory of transformism is, to all appearances, upon very strong ground. Its first strategic advantage over the theory of immutability consists in the fact that, unlike the latter, its attitude towards the problem is positive and not negative. When challenged to explain the structural uniformities observed in organic Nature, the theory of immutability is mute, because it knows of no second causes or natural agencies adequate to account for the facts. It can only account for homology by ascribing the phenomenon exclusively to the unity of the First Cause, and, while this may, of course, be the true and sole explanation, to assume it is tantamount to removing the problem altogether from the province of natural science. Hence it is not to be wondered at that scientists prefer the theory of transformism, which by assigning intermediate causes between the First Cause and the ultimate effects, vindicates the problem of organic origins for natural science, in assuming the phenomena to be proximately explicable by means of natural agencies. Asked whether he believes that God created the now exclusively arboreal Sloth (Bradypus) in a tree, the most uncompromising defender of fixism will hesitate to reply in the affirmative. Yet, in this case, what is nowadays, at least, an inherited preadaptation, dedicates the animal irrevocably to tree-life, and makes its survival upon the ground impossible.

Analogous preadaptations occur in conjunction with the phenomena of parasitism, symbiosis and commensalism, all of which offer instances of otherwise disparate and unrelated organisms that are inseparably bound together, in some apparently capricious and fortuitous respect, by a preadaptation of the one to the other. Parasites, guests, or symbiotes, as the case may be, they are now indissolubly wedded to some determinate species of host by reason of an appropriate and congenital adjustment. For all that, however, the association seems to be a contingent one, and it appears incredible that the associates were always united, as at present, by bonds of reciprocal advantage, mutual dependence, or one-sided exploitation. Yet the basis of the relationship is in each case a now inherited adaptation, which, if it does not represent the primitive condition of the race, must at some time have been acquired. For phenomena such as these, orthogenesis, which makes an organ the exclusive product of internal factors, conceiving it as a preformed mechanism that subsequently selects a suitable function, has no satisfactory explanation. Lamarckism, which asserts the priority of function and makes the environment mold the organ, is equally inacceptable, in that it flouts experience and ignores the now demonstrated existence of internal hereditary factors. But, if between these two extremes some evolutionary via media could be found, one must confess that it would offer the only conceivable “natural explanation” of preadaptation. All this, of course, is pure speculation, but it serves to show that here, at any rate, the theory of Transformism occupies a position from which it cannot easily be dislodged.

Vernon Kellogg has expressed this same view in a recent article, though he frankly admits that it is an as yet unrealized desideratum. “Altogether,” he says, “it must be fairly confessed that evolutionists would welcome the discovery of the actual possibility and the mechanism of transferring into the heredity of organisms such adaptive changes as can be acquired by individuals in their lifetime. It would give them an explanation of evolution, especially of adaptation, much more satisfactory than any other explanation at present claiming the acceptance of biologists.” (Atlantic Monthly, April, 1924, p. 488.)

But, besides the advantage of being able to offer a “natural explanation” of the association of homology with adaptation, Transformism enjoys the additional advantage of being able to make the imagination its partisan by means of a visual appeal. Such an appeal is always more potent than that of pure logic stripped of sensuous imagery. When it comes to vividness and persuasiveness, the syllogism is no match for the object-lesson. Retinal impressions have a hypnotic influence that is not readily exorcised by considerations of an abstract order—“Segnius irritant demissa per aurem, Quam quae sunt oculis subjecta fidelibus,” says Horace, in the “Ars Poetica.” Philosophers may distinguish between the magnetic appeal of a graphic presentation and the logical cogency of the doctrine so presented, but there is no denying that, in practice, imagination is often mistaken for reason and persuasion for conviction. Be that as it may, the ordinary method of bringing home to the student the evolutionary significance of homology is certainly one that utilizes to the full all the advantages of visual presentation. Given a class of impressionable premedics and coeds; given an instructor’s table with skeletons of a man, a flamingo, an ape and a dog hierarchically arranged thereon; given an instructor sufficiently versed in comparative osteology to direct attention to the points in which the skeletons concur: and there can be no doubt whatever as to the psychological result. The student forms spontaneously the notion of a common vertebrate type, and the instructor assures him that this “general type” is not, as it would be with respect to other subject matter, a mere universal idea with no formal existence outside the mind, but rather a venerable family likeness, posed for originally by a single pair of ancestors (or could it possibly have been, by one self-fertilizing hermaphrodite?) and recopied from generation to generation, with certain variations on the original theme, by the hand of an artist called Heredity. This explanation may be true, but logically consequential it is not. However, if the dialectic is poor, the pedagogy is beyond reproach, and the solution proposed has in its favor the fact that it accords well with the student’s limited experience. He is aware of the truism that children resemble their parents. Why look for more recondite explanations when one so obvious is at hand? The atavistic theory gratifies his instinct for simplification, and, if he be of a mechanistic turn of mind, the alternative conception of creationism is quite intolerable. Nevertheless, it goes without saying that the “inference” of common descent from the data of homology is not a ratiocination at all, it is only a simple apprehension, a mere abstraction of similarity from similars—“Unde quaecumque inveniuntur convenire in aliqua intentione intellecta,” says Aquinas, “voluerunt quod convenirent in una re.” (In lib. II sent., dist. 17, q. I, a. 1) Philosophy tells us that the oneness of the universal is conceptual and not at all extramental or real, but the transformist insists that the universal types of Zoölogy and Botany are endowed with real as well as logical unity, that real unity being the unity of the common ancestor.

Certainly, from the standpoint of practical effectiveness, the evolutionary argument leaves little to be desired. The presentation is graphic and the solution simple. But for the critic, to whom logical sequence is of more moment than psychological appeal, this is not enough. To withstand the gnawing tooth of Time and the remorseless probing of corrosive human reason, theories must rest on something sounder than a mirage of visual imagery!

Tell me where is fancy bred, Or in the heart or in the head? How begot, how nourished? Reply, reply. It is engendered in the eyes, With gazing fed; and fancy dies In the cradle where it lies.

But is it fair thus to characterize the “common ancestors” of Transformism as figments which, like all other abstractions, have no extramental existence apart from the concrete objects whence they were conceived? To be sure, their claim to be real entities cannot be substantiated by direct observation or experiment, and so a factual proof is out of the question. Man, the late-comer, not having been present at the birth of organic forms, can give no reliable testimony regarding their parentage. In like manner, no a priori proof from the process of inheritance is available, because heredity, as revealed to us by the experimental science of Genetics, can account for specific resemblances only, and cannot be invoked, at present, as an empirically tested explanation for generic, ordinal, or phyletic resemblances. It has still to be demonstrated experimentally that the hereditary process is transcendental to limits imposed by specific differentiation. There remains, however, the a posteriori argument, which interprets homology and adaptation as univocal effects ascribable to no other agency than the dual process of inheritance and variation. What are we to think of this argument? Does it generate certainty in the mind, or merely probability?

A moment’s reflection will bring to light the preliminary flaw of incomplete enumeration of possibilities. To suppose that inheritance alone can account for structural resemblance is an unwarranted assumption. Without a doubt, there are other similifying influences at work in Nature besides inheritance. True, inheritance is one possible explanation of the similarity of organisms, but it is not the only one. Even among the chemical elements of inorganic nature we find analogous uniformities or “family traits,” which, in the absence of any reproductive process whatever, we cannot possibly attribute to inheritance. Mendeléeff’s discovery of the periodicity of the elements, arranged in the order of their atomic weights, is well-known. At each interval of an octave, a succession of chemical types, similar to those of the preceding octave, recur. Hence elements appearing in the same vertical column of the Periodic Table have many properties in common and exhibit what may be called a family resemblance. Now, we have in the process of atomic disintegration, as observed in radioactive elements and interpreted by the electronic theory of atomic structure, a reasonably satisfactory basis upon which to account for the existence of these inorganic uniformities. Here analogous chemical constitution, produced in accordance with a general law, results in uniformity that implies a similar, rather than an identical, cause. The hypothesis of parallelistic derivation from similar independent origins accounts quite as well for the observed uniformities as does the hypothesis of divergent derivation from a single common origin. Why, then, should we lean so heavily on the already overtaxed principle of inheritance, when parallelism is as much a possibility in the organic world as it is an actuality in the inorganic world?

As to the contrast here drawn between inheritance and other similifying factors, it is hardly necessary to remark that we are speaking of inheritance as defined in terms of Mendelian experiment and cytological observation. In the so-called chemical theory of inheritance, the distinction would be meaningless and the contrast would not exist. Ehrlich’s disciple, Adami, sets aside all self-propagating germinal determinants, like the chromomeres, in favor of a hypothetical “biophoric molecule,” which is to be conceived as a benzine-like ring bristling with sidechains. Around this determining core the future organism is built up in definite specificity, as an arch is constructed about a template. Adami has merely applied Paul Ehrlich’s ideas concerning metabolism and immunity to the question of heredity, commandeering for this purpose the latter’s entire toolkit of receptors, haptophores, amboceptors, etc., as though this grotesque paraphernalia of crude and clumsy mechanical symbols (which look for all the world like the wrenches of a machinist, or the lifters used by the cook to remove hot lids from the kitchen range) could throw any valuable light whatsoever on the exceedingly complex, and manifestly vital, phenomenon of inheritance. It does not even deserve to be called a chemical theory, for, as Starling correctly remarks concerning Ehrlich’s conception, “though chemical in form,” it is not so in reality, because “it does not explain the phenomenon by reference to the known laws of chemistry.” (Cf. Physiology, ed. of 1920, p. 1084.) In a word, the theory of heredity, which seeks to strip inheritance of its uniqueness as a vital process by identifying it with the more general physicochemical processes occurring in the organism, is a groundless speculation, that, far from explaining, flouts the very observational data which it pretends to elucidate. Kurz und gut! to requite the mechanist, Schäfer, with his own Danielesque phrase, here, as elsewhere, the mechanists have succeeded in extracting from the facts, not what the facts themselves proclaim, but what preëxisted in their own highly-cultured imaginations so well-stocked with cogs, cranks, ball bearings, and other æsthetic imagery emanating from polytechnic schools and factories.

But in arguing from the existence of parallelism in the inorganic world to its possibility in the organic world, we are less liable to displease the mechanists than those other extremists, the neo-vitalists, who will be prone to deny all parity between living, and inanimate, matter. Fortunately, we are in a position to appease the scruples of the latter by referring to the facts of convergence as universally accepted evidence that the phenomenon of parallelism occurs in animate, no less than inanimate, nature. Admitting, therefore, that the laws of organic morphology are of a higher order than those which regulate atomic, molecular, and multimolecular structure, these facts attest, nevertheless, that parallelisms arise in organisms of separate ancestry which are due, not to heredity, but to the uniform action of universal morphogenetic forces. Hence general laws can be invoked to account for organic uniformities with the same right that they are invoked to account for resemblances existing between the various members of a chemical “family” like the Halogens. And why should this not be so? Organisms have much in common that transcends any possible scheme of evolution and that cannot be brought into alignment with the position arbitrarily assigned them in the evolutionary family-tree. They all originate as single cells. Their common means of growth and reproduction is mitotic cell division. This leads to the production of a somatella, among the protista, and of a soma differentiated by histogenesis into two or three primary tissues, among the metista. All these fundamental processes are strikingly uniform throughout the entire plant and animal world. In these universal properties of living matter, therefore, we have a common basis for general structural and organizational laws, which, though irreducible to the “common ancestors” of Transformism, is quite adequate to account for both the homologies and analogies of living matter. Accept this basis of general laws regulating the development of living matter, and there is no difficulty in seeing why the problems posed by exposure to analogous environmental conditions are solved in parallel fashion by organisms, irrespective of whether they are nearly, or distantly, related in the sense of morphology. Transformism, on the other hand, can only account for homology at the expense of convergence, and for convergence at the expense of homology. So far as a common ancestral basis is concerned, the two kinds of resemblance are, from the very nature of the case, irreducible phenomena.

It is only, in fact, by surrendering the principle that similarity entails community of origin, and by falling back on the suggested common basis of general laws, that Transformism makes room in its system for the troublesome facts of convergence. “It might be reiterated in passing,” says Dwight, “that this ‘convergence’ business is a very ticklish one. We have been taught almost word for word that resemblance implies relationship, or almost predicates it; but according to this doctrine it has nothing to do with it whatever.” (“Thoughts of a Cath. Anat.,” p. 190.) And in a subsequent chapter he says: “No very deep knowledge of comparative anatomy is needed for us to know that very similar adaptations for particular purposes are found in very diverse animals. The curious low grade mammal, the Ornithorhynchus, with a hairy coat and the bill of a duck, is a familiar instance. We all know that the whales have the general form of the fish, although they are mammals, and going more into details we know that the whale’s flipper is on the same general plan as that of the ancient saurians.... The origin of the eye, according to evolutionary doctrines, has been a very difficult problem, which gets worse rather than better the more you do for it. Even if we could persuade ourselves that certain cells blundered along by the lucky mating of individuals in whom they were a bit better developed than in the others till they came to form a most complicated organ of sight, it would be a sufficient tax on our credulity to believe that this could come off successfully in some extraordinary lucky species; but that it should have turned out so well with all kinds of vertebrates is really too much to ask us to swallow. But this is not all: eyes are very widely spread among different classes of invertebrates. More wonderful still, the eyes of certain molluscs and crustacea are on stalks, and this is found also in various and very different families of fishes. How did this happen? Was it by way of descent from the molluscs or the crustacea? If not, how could chance have brought about such a similar result in diverse forms?” (Op. cit., pp. 233-236.)

It may be objected that the resemblances of convergence are superficial analogies, not to be confounded with fundamental homologies. This contention may be disputed; for, as we shall see in the next chapter, there are cases where the convergence is admittedly radical, and not merely superficial. The distinction, moreover, between shallow and basic characters is somewhat arbitrary, and its validity is often questionable. When the skeletal homology that relates the amphibia to the mammals, for instance, is traced to the root of the vertebrate family tree, we find it all but disappearing in a primitive Amphioxus-like chordate, whose so-called skeleton contains no trace of bone or cartilage. Hence, if we go back far enough, the homologies of today become the convergences of a geological yesterday, and we find the vertebrate type of skeleton arising independently in reptiles, mammals, amphibia, and fishes.

Again, there are times when convergent analogies appear to be more representative of the common racial heritage than the underlying structure itself, tempting the evolutionist to fly in the face of the orthodox interpretation, which rigidly rules out analogy in favor of homology, and refuses to accept the eloquent testimony of a remarkable resemblance merely because of a slight technical discrepancy in the structural substrate. A large pinching claw, or chela, for example, occurs in two organisms belonging to the phylum of the arthropods, namely, the lobster and the African scorpion. Both chelæ are practically identical in structure, but, unfortunately, the chela of the lobster arises from a different appendage than that from which the scorpion’s chela emerges. If they arose from corresponding appendages, they would be pronounced “homologous organs” and acclaimed, without hesitation, as strong evidence in favor of the common origin of all the arthropods. In proof of this, we call attention to the importance attached to the adaptations affecting homologous bones in fossil “horses.” As it is, however, the two chelæ are analogous, and not homologous, organs. Hence, technically speaking, the two chelæ are utterly unrelated structures. To the eye of common sense, however, the likeness appears to be far more important than the difference, and the average person will be inclined to view the resemblance as evidence of a community of type. In fact, the tendency to discard superficial, and to retain only fundamental, uniformities, is dangerous to the theory of Transformism. When we confine our attention to what is really basic, we find that the resemblances become so generalized and widespread that specific conclusions as to descent become impossible, and we lose all sense of direction in a clueless labyrinth of innumerable, yet mutually contradictory, possibilities.

Finally, it may be noted in passing that, though it is customary with evolutionists to regard homologous characters as the tenaciously persistent heritage of primeval days, and to look upon adaptational characters as adventitious and accessory to the aforesaid primitive heritage, the supposedly older and more fundamental characters fail to give, by the manifestation of greater fixity, any empirical evidence whatever of their being more deeply or firmly rooted in the hereditary process than the presumably newer adaptational characters. We have, therefore, no experimental warrant for appropriating homologous, rather than adaptational, characters to the process of inheritance. “It is sometimes asserted,” says Goodrich, “that old-established characters are inherited, and that newly begotten ones are not, or are less constant, in their reappearance. This statement will not bear critical examination. For, on the one hand, it has been conclusively shown by experimental breeding that the newest characters may be inherited as constantly as the most ancient.... While, on the other hand, few characters in plants can be older than the green color due to chlorophyll, yet it is sufficient to cut off the light from a germinating seed for the greenness to fail to appear. Again, ever since Devonian times vertebrates have inherited paired eyes; yet, as Professor Stockard has shown, if a little magnesium chloride is added to the sea water in which the eggs of the fish Fundulus are developing, they will give rise to embryos with one median cyclopean eye! Nor is the suggestion any happier that the, so to speak, more deep-seated and fundamental characters are more constantly inherited than the trivial or superficial. A glance at the organisms around us, or the slightest experimental trial, soon convinces us that the apparently least important character may reappear as constantly as the most fundamental. But while an organism may live without some trivial character, it can rarely do so when a fundamental character is absent, hence such incomplete individuals are seldom met in Nature.” (Science, Dec. 2, 1921, p. 530.)

But, whether it be upon, or beneath, the surface, similitude of any kind suffices to establish our contention that inheritance is not the only similifying influence present in organisms, and that resemblance is perfectly compatible with independence of ancestry. We have, therefore, an alternative for inheritance in the explanation of organic uniformities, and by the admission of this alternative, which, for the rest, is factually attested by the universally acknowledged phenomena of convergence, the inference of common descent from structural resemblance is shorn of the last remnant of its demonstrative force, as an a posteriori argument.

But a still more serious objection to the evolutionary interpretation of homology and preadaptation arises from its intrinsic incoherency. Evolution, as previously stated, is assumed to be the resultant of a twofold process, namely, inheritance and variation. The first is a conservative and similifying process, which transmits. The second is a progressive and diversifying process, which diverts. To the former process are due the uniformities of homology, to the latter the deviations of adaptation. Upon the admission of evolutionists themselves, however, neither of these processes behaves in a manner consistent with its general nature, and both of them are flagrantly unfaithful to the principal rôles assigned to them. Nowadays the hereditary process transmits adaptational, as well as homologous, characters. If, then, adaptational characters are more recent than homologous characters, there must have been a time when inheritance ceased to similify and become a diversifying process by transmitting what it did not receive from the previous generation. There were times when, not content with simply reiterating the past, it began to divert former tendencies into novel channels. In other words, inheritance becomes dualized into a paradoxical process, which both perpetuates the old and appropriates the new. The same inconsistency is manifest in the process of variation, which capriciously produces convergent, no less than divergent, adaptations. In two fundamentally identical structures, like the wing of a bird and the foreleg of a cat, variation is said to have produced diverse adaptations. In two fundamentally diverse structures, like the head of an octopus and the head of a frog, variation is said to have produced an identical adaptation, namely, the vertebrate type of eye. It appears, therefore, that the essentially diversifying process of variation can become, on occasion, a simplifying process, which, instead of solving environmental problems in an original manner, prefers to employ uniform and standardized solutions, and to cling to its old stereotyped methods. Inheritance similifies and diversifies, variation converges and diverges. It is futile to attempt to reduce either of these protean processes to a condition that even approximates consistency. The evolutionist blows hot and cold with the same breath. Verily, his god is Proteus, or the double-headed Janus!

Summa summarum: The evolutionary argument from homology is defective in three important respects: (1) in its lack of experimental confirmation; (2) in its incomplete enumeration of the disjunctive possibilities; (3) in its inability to construct a scheme of transmutation that synthesizes inheritance and variation in a logically coherent, and factually substantiated formula. The first two defects are not necessarily fatal to the argument as such. Though they destroy its pretensions to conclusiveness, they do not preclude the fulfilment of the moderate claim made in its behalf by Prof. T. H. Morgan, who says: “In this sense (i.e., as previously stated) the argument from comparative anatomy, while not a demonstration, carries with it, I think, a high degree of probability.” (“A Critique of the Theory of Evolution,” p. 14.) The third defect is more serious. The apparently irreducible antagonism which the evolutionary assumption introduces between inheritance and variation has been sensed even by the adherents of transformism themselves, and they have searched in vain for a formula, which, without sacrificing the facts, would bring into concord the respective rôles of these discordant factors. “It follows,” says Osborn, “as an unprejudiced conclusion from our present evidence that upon Weismann’s principle we can explain inheritance but not evolution, while with Lamarck’s principle and Darwin’s selection principle we can explain evolution, but not, at present, inheritance. Disprove Lamarck’s principle and we must assume that there is some third factor in evolution of which we are ignorant.” (Popular Science Monthly, Jan., 1905.) The point is well taken, and unless, as Osborn suggests, there is a tertium quid by means of which the discord can be resolved into ultimate harmony, we see no way of liberating the theory of Transmutation from this embarrassing dilemma.