EVIDENCES FROM MORPHOLOGY 131 
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. 13). 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 life. 
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. 14). 
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 
