Aim and Method of Comparative Anatomy .'581 



produced S, a spiracle. Along other lines s became s a , which produced 

 S a , the simple middle-ear cavity of some amphibians — frogs and toads. 

 Later s a became s or , the germinal basis for the modified middle-ear 

 cavity (S ar ) of a reptile, and, still later, became s arm which produces 

 S arm , the more complex middle-ear cavities of a mammal. The evidence 

 is admittedly indirect, but it is the best obtainable. 



Charles Darwin regarded embryonic origin as most important, or 

 even sufficient in itself, as a criterion for homology. In the Glossary of 

 his "Origin of Species," he defines homology as "That relation between 

 parts which results from their development from corresponding em- 

 bryonic parts, either in different animals, as in the case of the arm of 

 man, the foreleg of a quadruped, and the wing of a bird ; or in the same 

 individual, as in the case of the fore and hind legs inquadrupeds. ..." 



Discussion of the criteria for homology leads to this conclusion: 

 In establishing homology of organs, all available data must be carefully 

 considered. No one characteristic is trustworthy by itself. Probably 

 embryonic origin is most important, but its evidence is inevitably 

 circumstantial and involves some large assumptions. Relative posi- 

 tion and connections of parts are always highly important. Structure 

 — internal differentiation — is significant, but the close alliance of 

 structure with function may lead to deceptive similarity where no 

 homology exists. External form of organs usually has little significance. 

 Number of multiple constituent parts of organs is rarely significant ; 

 size and color still less so. Function is quite independent of homology. 

 Organs having the same function are analogous; they may or may not 

 be homologous. 



"Conservatism" 



In judging homology of organs, more weight is given to some char- 

 acteristics than to others. Similarly, in using homologies as the basis 

 for judging genetic relationship of animals, some organs, or systems 

 of organs, are more significant than others. The findings of compara- 

 tive anatomy make it clear that some organs are much more readily 

 changed than others. An organ which is subject to rapid (in terms of 

 evolutionary time) and radical change provides a poor clue to the 

 anatomist who is trying to thread his Avay through the maze of ances- 

 try. The organ becomes unrecognizable, or even disappears, and 

 he loses the trail. An organ which is little prone to change may guide 

 him safely into the very remote past. Organs of the latter sort are 

 called "conservative." 



If we were to judge the relationships of vertebrates on the basis of 

 the skin only, we would find no good reason for thinking that reptiles, 

 birds, and mammals were related to amphibians and fishes. The charac- 



