THE UNDERLYING IDEA 59 
animals which meets such a calamity has before it 
only three possible outcomes of the struggle. First 
it may be plastic enough and it may vary enough in 
the right direction to adjust itself to the changed con- 
ditions. In this case it and a favored few like it 
will occupy the altered territory. The second possi- 
bility is that it may migrate while the actual change is 
going on, thus remaining in the sort of situation 
suited to it and its kind. The third possibility is the 
one which overtakes a great majority of animals— 
they die. Even the entire line dies out, and the strata 
of the rocks are filled with the bones, shells, and teeth 
of such as have met this fate. They have become 
extinct. 
Thus far in this chapter we have been considering 
the influences under which it is conceivable that ani- 
mals should advance. There is no question whatever 
that there are too many animals born, nor is there any 
possible question that a very large proportion of them 
must certainly die. There is equally no doubt that 
every animal produces after its own kind, and that its 
offspring, while they resemble it closely, still vary a 
little from it and from each other. This fact is per- 
fectly plain to the most superficial observer who 
thinks on the matter at all. It is not so plain, nor is 
it easily demonstrated, that all of these acting to- 
gether do surely, even if slowly, alter the form and 
