370 



Comparative Anatomy — Its History, Aim, and Method 



Fig. 294. Convergence in modification of hairs to become stiff, sharp spines. 

 (Top) Porcupine, a rodent. (Bottom, left) Hedgehog, an insectivore. (Bottom right) 

 Spiny anteater, a monotreme (Top, Courtesy, American Museum of Natural 

 History, New York. Bottom, Courtesy, Museum of Comparative Zoology, Harvard 

 University.) 



modern bats (Fig. 439). But, so far as we know, the earliest mammals 

 were all nonflying. The flying membranes of these several animals must 

 have had independent origin — admitting, however, a remotely possible 

 genetic relation between the wings of the bat and the flying membranes 

 of Galeopithecus. 



Criteria of Homology 



The method is comparison. The parts to be compared must be re- 

 lated by genetic descent — homologous parts. Relationship between two 

 animals is to be judged according to the degree of similarity between 

 their homologous organs. The great difficulty which the anatomist en- 

 counters lies in the fact that so many homologies are not self-evident. 

 Homology may be obscured by profound structural difference of the 



