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?