Aim and Method of Comparative Anatomy 363 



The term "homomorphic" has been applied to such things as 

 hairs, which are similar in general structure but have no definite rela- 

 tion to axes of the body. 



These names which have been applied to the several types of 

 "general homology'' are not important, but they have been mentioned 

 as a means of emphasizing the fact that "repetition of parts" is a com- 

 mon and important feature of animal structure and that the occurrence 

 of such repetition in various axial relationships results in internal 

 correspondences of definitely different sorts. 



History shows that the concept of homology has undergone a slow 

 evolution. To the anatomists of a century ago, homology (or "anal- 

 ogy," indifferently) was a correspondence based on function, structure, 

 position, connections, and anything else. To Owen, it was a vaguely 

 defined set of structural (but not functional) resemblances which led 

 him to feel justified in giving the "same name" to organs of two differ- 

 ent animals. It is as if by an afterthought that he suggests that inherit- 

 ance is probably at the bottom of his "namesakeism." Charles Darwin 

 and Haeckel realized clearly enough that homologies depend upon 

 inheritance. But now it has become quite clear that the idea of Homol- 

 ogy, if Comparative Anatomy is to use it as the tool for digging out 

 the histories of races of animals, must be rigidly restricted to genetic 

 correspondence of parts. This necessity is recognized in the admirable 

 definition offered by T. H. Montgomery, Jr.* (University of Pennsyl- 

 vania), in 1902. He defined homology as "a relation of genetic 

 descent." Hatschek, in 1888, expressed the same idea but in many 

 more words. 



It is true that homologous organs are usually similar in many or 

 even in all respects, including function. But Montgomery's definition 

 recognizes the possibility that two organs which are different in all 

 particulars are the same organ if they can be traced to common genetic 

 origin. Resemblances in form, structure, position, and connections may 

 point to the conclusion that two organs are homologous, but such 

 resemblances, in themselves, do not constitute homology. 



It is necessary that the "relation of genetic descent" be clearly 

 understood. It does not involve any direct relation of one organ to 

 another. A brain is not descended from the brain of an ancestor, 

 whether remote or immediate. The relation between organs which are 

 "the same" in animals of successive generations is an indirect one 

 depending on the production of successive individuals from a continu- 

 ous germ-plasm (Fig. 290). In a strict sense, a metazoan animal, repro- 



* Montgomery, T. H., Jr.: On Phylogenetic Classification, Proc. Acad. Natur. 

 Sc. 54:187-232, Philadelphia, 1902; The Analysis of Racial Descent in Animals, 

 New York, Henry Holt and Co., 1906. 



