THE STRUGGLE FOR EXISTENCE 407 



undergo the struggle and the selecting process, and again 

 the fittest live. And so on until adjustment or harmoniz- 

 ing of animals' bodies and habits with the conditions of 

 life, with their environment, comes to be extremely fine 

 and nearly perfect. 



It is evident, of course, that such a natural selection or 

 survival of the fittest and consequent adaptation to en- 

 vironment presupposes differences among the individuals 

 of a species. And this is an observed fact. No two 

 individuals, although of the same brood, are exactly alike 

 at birth ; there always exist slight variations in structure 

 and performance of functions. And these slight variations 

 are the differences which determine the fate of the indi- 

 vidual. One individual is a little larger or stronger or 

 swifter or hardier than its mates. The existence of this 

 variation we know from our observation of the young 

 kittens or puppies of a brood. So it is with all animals. 

 Thus natural selection depends upon two factors, namely, 

 the excess in the production of new individuals and the 

 consequent struggle for existence among them, and the 

 existence of variations which give certain individuals 

 slight advantages in this struggle. 



Adaptation and adjustment to surroundings. The 

 action of natural selection obviously must, and does, 

 result in a fine adaptation and adjustment of the structure 

 and habits of animals to their surroundings. If a certain 

 species or group of individuals cannot adapt itself to its 

 environment, it will be crowded out by others that can. 

 A slight advantageous variation comes in time by the 

 continuously selective process to be a well-developed 

 adaptation. 



The diverse forms and habits possessed by animals are 

 chiefly adaptations to their special conditions of life. 

 The talons and beak of the eagle, the fishing-pouch of the 

 pelican, the piercing chisel-like bill of the woodpecker, 



