18 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 
