go THE MEANING OF EVOLUTION 
its present surroundings, it must either meet the diffi- 
culty by altering itself, or it must give up the strug- 
gle. The alteration is unconscious so far as the ani- 
mal is concerned. It is seriously to be doubted 
whether there is any recognition of the process on the 
part of any animal excepting man. But though the 
process be unconscious, it is none the less there. 
Slowly and gradually the animal and the environment 
are becoming adjusted to each other. 
While it is exceedingly difficult to lay our hands on 
any animal which is at present visibly changing its 
structure, it is not hard to find closely related animals. 
These are nearly alike in structure in most respects. 
In a few points, however, they may differ materially, 
and these points are often directly concerned with dif- 
ferent habits of life. Considered in this aspect, these 
adaptations of a single organ separately examined 
form an excellent argument in favor of that gradual 
alteration of the entire organism which evolution sug- 
gests. 
The most primitive struggle in which an animal can 
possibly engage is the effort to maintain its own life 
and vigor. This struggle will result in certain adapta- 
tions for the individual, adjustments which make for 
the safety of the animal himself. These form the sub- 
ject matter of the present chapter. 
The farther up the animal kingdom we pass in the 
