68 EVOLUTION, GENETICS, AND EUGENICS 



those parts of the organism which it was for the benefit of the organism 

 to have altered, so that it might be adapted to an aquatic mode of 

 existence. Thus the arm, which is used as a fin, still retains the bones 

 of the shoulder, fore-arm, wrist, and fingers, although they are all 

 enclosed in a fin-shaped sack, so as to render them useless for 

 any purpose other than swimming (Fig. 3). Similarly, the head, 

 although it so closely resembles the head of a fish in shape, still retains 

 the bones of the mammalian skull in their proper anatomical relations 

 to one another; but modified in form so as to offer the least possible 

 resistance to the water. In short, it may be said that all the modifi- 

 cations have been effected with the least possible divergence from the 

 typical mammalian type, which is compatible with securing so perfect 

 an adaptation to a purely aquatic mode of Ufe. 



Now I have chosen the case of the whale and porpoise group, 

 because they offer so extreme an example of profound modification of 

 structure in adaptation to changed conditions of life. But the same 

 thing may be seen in hundreds and hundreds of other cases. For 

 instance, to confine our attention to the arm, not only is the limb 

 modified in the whale for swimming, but in another mammal — the 

 bat — it is modified for flying, by having the fingers enormously 

 elongated and overspread with a membranous web. 



In birds, again, the arm is modified for flight in a wholly different 

 way — the fingers here being very short and all run together, while the 

 chief expanse of the wing is composed of the shoulder and forearm. 

 In frogs and lizards, again, we find hands more like our own; but in 

 an extinct species of flying reptile the modification was extreme, the 

 wing having been formed by a prodigious elongation of the fifth finger, 

 and a membrane spread over it and the rest of the hand (Fig. 4). 

 Lastly, in serpents the hand and arm have disappeared altogether. 



Thus, even if we confine our attention to a single organ, how 

 wonderful are the modifications which it is seen to undergo, although 

 never losing its typical character. Everywhere we find the distinction 

 between homology and analogy which was explained in the last 

 chapter — the distinction, that is, between correspondence of structure 

 and correspondence of function. On the one hand, we meet with 

 structures which are perfectly homologous and yet in no way 

 analogous; the structural elements remain, but are profoundly 

 modified so as to perform wholly different functions. On the other 

 hand, we meet with structures which are perfectly analogous, and 

 yet in no way homologous; totally different structures are modified 



