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 



