XX.] COMPARATIVE MORPHOLOGY. 245 



In a precisely similar manner, organs in different species, 

 which are plainly homologous as to their position with 

 respect to the rest of the body, and the number and re- 

 lative position of their parts, are modified to serve totally 

 unlike purposes in life. Thus, within the boundaries of 

 the single class of the Mammalia, the hand of the man, Haud,foot, 

 the fore-foot of the dog, the wing of the bat, and the paddle padfie^aU 

 .of the whale, are all homologous ; and not only homo- bomolo- 

 logous in position and in general relation to the spine and ^°"*" 

 the rest of the skeleton, but down to the minutest detail : 

 every bone in one has its homologous or corresponding 

 bone in the others, though they are all variously modified 

 in shape and size, to suit the purposes of the various 

 modes of life of the animals to which they belong. Thus, 

 if we would know all that is to be known about the 

 various forms of vertebrate skeleton, we must understand, 

 not only the manner in which the skeleton of each species 

 is modified in order to adapt it to its mode of life, but also 

 the homological resemblances that underlie the adaptive 

 modifications. To express the same by an example : if we 

 would know all that is to be known about the man's 

 hand and the bat's wing, we must understand the resem- 

 blance, amounting to identity, between the number and 

 relative position of the bones of both, as well as the struc- 

 tural differences which adapt the hand for gxasping and 

 the wing for flight. The law of the adaptation of structure Adapta- 

 to function will not account for this homological resem- ^^ ^'^^ 

 blance — not only general, but in detail — between organs account 



ji , T^ • jy J.- for liomo- 



tnat are unlike m lunction. loo-y. 



We see the same combination of two mutually modify- 

 ing principles — morphological resemblance and adaptive 

 difference — in comparing the vertebrae of different animals. 

 Among fishes and serpents these are distinct, and jointed Vertebra 

 the one to the other, so as to be moveable. Among Mam- theTwer^ 

 malia and birds, some of the vertebrae continue to be Vertebrata, 

 moveable, but the form of the body, and the relation of the united 

 limbs to it, make it necessary for other vertebrae to be \^} *|'® 

 immoveably fastened together. Even when this is the case, 

 however, they retain their visible distinctness as vertebrae ; 



