Chap. I.] HOMO LOGY. II 



The reason for this is plain, the moment we clearly 

 understand what object the comparative anatomist 

 has before him ; it is that of coming to some general 

 conclusions as to identity or community of structure ; 

 for this purpose, then, he is not to compare parts 

 that have the same function, but those that are formed 

 in the same kind of way. The physiologist, on the 

 other hand, looking at organs as parts of a machine, 

 examines together those that do the same thing. 

 When we compare parts morphologically, we must not 

 be content merely with an analogy between them, 

 we must be careful that there is a homology or 

 real resemblance. 



The first criterion of homological parts is their 

 development from similar embryonic structures ; such 

 are the wing of a bird and the leg of a horse. But 

 a further question now arises ; why have these wings 

 and legs, which in their completed condition are so 

 different from one another, a similar structure in the 

 embryo 1 The answer to this is given by the doctrine 

 of descent, which supposes that the bird and the 

 horse had in the past a common ancestor, provided 

 with limbs simpler in structure than those of either 

 bird or horse, but having essentially that which they 

 have now, or which they have had, and from which 

 they are both derived ; a true and complete homology 

 of parts is, then, only to be found between animals 

 which have had a common ancestor provided with the 

 part to be compared. This complete homology may 

 be conveniently spoken of as homogeiiy (Ray 

 Lankester). 



It is very necessary to have before the mind this 

 idea of community of descent, because we shall con- 

 stantly meet with cases in which, with a very close re- 

 semblance in structure and mode of development, there 

 is not a complete identity in descent. For example, 

 all mammals and all birds are provided with four 



