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?