COMPARATIVE ANATOMY AND MORPHOLOGY 341 



most remote times. Aristotle formulated it clearly. Nevertheless, it 

 has been established scientifically, with the necessary details, for the 

 first time by E. Geoffroy Saint-Hilaire, under the name of the princi- 

 ple of analogy. 



Extending over the whole of the animal kingdom, it is true only 

 for very general and very vague structures. If we go into details, we 

 find important divergences, and it is only by forcing and falsifying 

 comparisons that we can succeed in establishing apparent analogies 

 between organizations thoroughly unlike. The nervous system of a 

 mollusk has almost no resemblance to that of a vertebrate; the 

 organization of a sea-urchin has almost nothing in common with that 

 of an ascidian; the olfactory organ of a crab does not resemble in 

 any possible way that of a mammal. 



It is not the same, however, if, in place of extending the comparison 

 to the whole of the animal kingdom, we limit it to a small number of 

 large groups. It is the merit of Cuvier that he established these 

 groups by means of comparative anatomy; they are the phyla of the 

 animal kingdom. 



In any one phylum the organization of all the beings which com- 

 pose it is truly very similar, and if there are differences, real, various, 

 and profound, they are not incapable of reduction. 



This principle furnishes to comparative anatomy a third object; it 

 points out a third aim more elevated than the first two, and more 

 difficult to attain. Here not only does comparative anatomy become 

 an independent science because of this object, but it opens to research 

 an unlimited field. It is no longer, as in its first aspect, the simple 

 complement of another science; it is no longer, as in its second aspect, 

 a science limited to the discovery of some rare general principles; it 

 acquires a new dignity, and it presents itself as so vast that its study 

 will never be achieved. 



This third aim of comparative anatomy is the comparison of all the 

 beings belonging to the same type, it is the search for the fundamental 

 conformity under the divergences of detail. 



This fundamental conformity makes it possible to conceive for each 

 phylum a type of structure from which is derived, as a modifica- 

 tion of the type, the structure of each of the forms constituting the 

 phylum. 



If all the organisms of the same phylum are derived from a single 

 type, the organs of each one of them are derived from the organs of 

 the type. Therefore, whatever may be the differences presented by 

 corresponding organs of two members of the phylum, these differences 

 are secondary, accidental, subordinate to a fundamental conformity, 

 and the organs, representing the same organ of the type, represent 

 one another and are homologous. 



The chief and the highest aim, therefore, of comparative anatomy 



