342 COMPARATIVE ANATOMY 



becomes the search for homologies. The science of homologies, or 

 morphology, is considered the essential part of comparative anatomy, 

 at once the noblest and the most fruitful. 



In the comparison of organs of different beings, Geoffroy Saint- 

 Hilaire and his contemporaries took into account all the character- 

 istics, physiological as well as anatomical. Thus they were not afraid 

 to compare the lungs of mammals with the gills of fishes. To R. 

 Owen is due the credit of distinguishing between physiological and 

 anatomical characteristics; he pointed out that the former furnish 

 only superficial analogies, while the latter form the true basis of 

 homology. Thus the wing of an insect and the wing of a bird are 

 analogous as being organs of flight, but they are by no means homo- 

 logues, having an essentially different structure, since the wing of a 

 bird is the homologue of the arm of man, for we find in it, in modi- 

 fied form but with similar arrangement, the bones of the arm, the 

 forearm, and the hand, and a goodly number of muscles which serve 

 to move it. 



It is evident, then, that morphology depends entirely upon the idea 

 of an animal type. The solution, therefore, of morphological problems 

 depends upon the idea of a uniformity in the structure of beings and 

 upon the significance of types. 



Now these conceptions vary. The Deists, and therefore Aristotle, 

 explain uniformity of structure by unity of plan. A creative God 

 created species, and by an act of his will constructed them according 

 to a uniform plan, varying only in details and in the application of its 

 fundamental facts. 



The aim of comparative anatomy is to discover this plan, either by 

 the study of animals or by rethinking the thought of God. 



Others, in a conception more or less pantheistic, attribute to nature 

 what the preceding had made proceed directly from God. There is 

 still a plan, but an unconscious one, or rather a model, a type, an 

 immaterial entity, unrealized, but which yet controls the realization 

 of real forms as the laws of nature direct the phenomena which are 

 subject to them. 



This type is understood in two ways. According to one, it is a pro- 

 totype, the original form, of which real beings are gradual improve- 

 ments; according to the other, it is an archetype, the perfect form, of 

 which existing creatuies are the repeated models, infinitely various, 

 but always degraded, degenerate. 



The aim of comparative anatomy is to find this type and to deter- 

 mine how nearly real forms approach to it. 



This conception of types, apparently less childish than that of the 

 unity of plan, has been in fact more troublesome; for, because of its 

 greater philosophical attraction, it has seduced more and higher intel- 

 lects and has directed their efforts towards an end not less chimerical. 



