1 8 Evolution and Adaptation 



the term includes a large number of phenomena of very 

 different kinds is apparent. When we have examined these 

 phenomena further we shall find, I think, that it will be nec- 

 essary to put some of them into different categories and 

 treat them differently. It is probably incorrect to suppose 

 that all processes useful to the organism have been acquired 

 in the same way, nevertheless, for the present the term 

 adaptation is sufficiently general, even if vague, to cover 

 these different groups of cases. 



It may be asked, in what respects are these structures 

 and processes of adaptation different from the ordinary struc- 

 tures and changes that go on in the organism ? Why is the 

 leg of the mole more of an adaptation than that of a dog ? 

 The one is of as much use as the other to its possessor. 

 What reason can we give for citing the poison of the snake, 

 and not mentioning in the same connection the other glands 

 of the body ? In fact, the poison gland of the snake is sup- 

 posed to be a modified superior labial gland. Why, in short, 

 are not the processes of digestion, excretion, secretion, the 

 beating of the heart, the ordinary reflex acts of the nervous 

 system, and the action of the sense organs, as truly adapta- 

 tions as the special cases that have been selected for illustra- 

 tion. The answer is simply that we are more impressed by 

 those cases of adaptation that are more unusual, as when an 

 animal departs in the use of certain structures from the rest 

 of the group to which it belongs. For example, if all mam- 

 mals lived underground, ourselves included, and the fore-legs 

 or arms were used for burrowing, we should not think this 

 unusual ; but if we found an animal using all four legs to 

 support the body and for purposes of progression, we 

 should, most likely, think this was an excellent illustration of 

 adaptation. 



In other instances the condition is somewhat different. 

 The color of certain animals may unquestionably be of use 

 to them in concealing them from their enemies. In other 



