COMPARATIVE ANATOMY AND MORPHOLOGY 343 



The introduction into biology of the concept of descent has pro- 

 duced an important change: the ideal prototype has become an 

 objective reality in the form of the ancestral type. It is certain that 

 two given forms, if, at least, they are not too different from one an- 

 other, have common ancestors, of which the latest is that one from 

 which they differ least. This latest ancestor is the material prototype 

 from which they have both really been derived. The organs of the 

 ancestor have truly become the organs of the two derived forms as 

 far as we may consider as one and the same thing the organs of one 

 being and the almost identical ones of its immediate descendant. 

 We, then, have the right to say that from the phylogenetic point of 

 view the organs of the later forms represent those of the ancestor, and 

 that those of their organs which represent the same organ of the 

 ancestor represent those of one another. The idea of representation 

 gains body and becomes a reality, and morphology, which is the 

 science of representations, becomes a positive science. 



The aim of morphology therefore becomes precise and more pos- 

 itive. It consists in determining homologies, considering as homo- 

 logues those organs which in the ancestor were represented by a 

 common rudiment. 



Morphology, then, has phylogeny as its basis. But phylogeny is not 

 a science of direct observation; it is constructed inductively from 

 the facts of comparative anatomy, of paleontology, and of compara- 

 tive embryology. 



The theme of morphology is, then, as follows: to compare organs 

 which we suppose may be homologues and which belong to two differ- 

 ent species, to determine by comparative observations, anatomical, 

 paleontological, and embryological, not all of the characteristics of 

 the ancestor of the tw T o species, which is the aim of pure phylogeny, 

 but the typical constitution of that organ in the common ances- 

 tor, and to see whether the two organs have surely been derived 

 from the ancestral type. In that case they are called homologous, 

 whatever may be their differences; in the contrary case they are 

 not homologous, whatever may be their resemblances. 



This is the matter in its brutal clearness. 



Let us take an example: what, in the foot of the ox, is the homo- 

 logue of the hoof of the horse? The answer is not evident. A philo- 

 sopher of nature could well conceive an archetype according to which 

 the whole of the foot of the ox corresponds to the hoof of the horse. 

 But the anatomist dissects these parts and finds only one digit in 

 the foot of the horse while he finds two in that of the ox. The em- 

 bryologist sees the five digits of the unguiculates shown at first in 

 the horse and the ox; digit number one disappears first, afterwards 

 numbers two and five; the ox preserves three and four; in the 

 horse number four disappears in its turn, leaving only the middle 



