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.