376 Evolution and Adaptation 
or even injurious to a slight degree, it might have been 
retained, as appears in fact to be the case in the change of 
color of the green lizards. 
INCREASE OF ORGANS THROUGH USE AND DECREASE 
THROUGH DISUSE 
We meet here with one of the most characteristic and 
unique features of living things as contrasted with non-living 
things. We shall have to dismiss at once the idea that we 
can explain this attribute of organisms by either the selec- 
tion or the mutation theory; for we find animals possessing 
this power that could never be supposed to have acquired it 
by any experience to which they have been subjected; and 
since it appears to be so universally present, we cannot 
account for it as a chance mutation that may have appeared 
in each species. No doubt Wolff had responses of this kind 
in view when he made the rather sweeping statement that 
purposeful adaptation is the most characteristic feature of 
living things. The statement appears to contain a large 
amount of truth, if confined to the present group of phe- 
nomena. 
This power of self-regulation may confer a great benefit 
on its possessor. The increase in the size and strength of 
the muscles through use may give the animal just those 
qualities that make its existence easier. The increase in the 
power of vision, or at least of visual discrimination through 
use, of the power of smell and of taste, of hearing and of 
touch, are familiar examples of this phenomenon. 
However much we may be tempted to speculate as to how 
this property of the animal may have been acquired, we lack 
the evidence which would justify us in formulating even a 
working hypothesis. It may be that when we come to know 
more of what the process of contraction of the muscle in- 
volves, the possibility of its development as a consequence 
