EVOLUTION AND ADAPTATION 
CHAPTER I 
THE PROBLEM OF ADAPTATION 
BETWEEN an organism and its environment there takes 
place a constant interchange of energy and of material. This 
is, in general, also true for all bodies whether living or lifeless ; 
but in the living organism this relation is a peculiar one; first, 
because the plant or the animal is so constructed that it is 
suited to a particular set of physical conditions, and, second, 
because it may so respond to a change in the outer world 
that it further adjusts itself to changing conditions, ze. the 
response may be of such a kind that it better insures the 
existence of the individual, or of the race. The two ideas 
contained in the foregoing statement cover, in a general way, 
what we mean by the adaptation of living things. The fol- 
lowing examples will serve to illustrate some of the very 
diverse phenomena that are generally included under this 
head. 
STRUCTURAL ADAPTATIONS 
‘The most striking cases of adaptations are those in which 
a special, in the sense of an unusual, relation exists between 
the individual and its surroundings. For example, the fore- 
leg of the mole is admirably suited for digging underground. 
A similar modification is found in an entirely different group 
of the animal kingdom, namely, in the mole-cricket, in which 
the first legs are also well suited for digging. By their use the 
mole-cricket makes a burrow near the surface of the ground, 
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