406 ELEMENTARY ZOOLOGY 



The struggle for existence. The numbers of animals 

 are stationary because of the tremendous mortality occa- 

 sioned by the constant preying on eggs and young and 

 adults by other animals, because of strenuous and destruc- 

 tive climatic and meteorological conditions, and because 

 there is not space and food for all born, not even, indeed, 

 for all of a single species, let alone all of the hundreds of 

 thousands of species which now inhabit the earth. There 

 is thus constantly going on among animals a fearful 

 struggle for existence. In the case of any individual this 

 struggle is threefold: (i) with the other individuals of his 

 own species for food and space; (2) with the individuals 

 of other species, which prey on him, or serve as his prey, 

 or for food and space; and (3) finally with the conditions 

 of life, as with the cold of winter, the heat of summer, or 

 drouth 'and flood. Sometimes one of these struggles is 

 the severer, sometimes another. With the communal 

 animals the struggle among individuals is lessened they 

 help each other ; but when the struggle with the condi- 

 tions of life are easiest, as in the tropics or in the ocean, 

 the struggle among individuals becomes intensified. Each 

 strives to feed itself, to save its own life, to produce and 

 safeguard its young. But in spite of all their efforts only 

 a few individuals out of the hosts produced live to 

 maturity. The great majority are destroyed in the egg 

 or in adolescence. 



Variation and natural selection. What individuals 

 survive of the many which are born ? Those best fitted 

 for life; those which are a little stronger, a little swifter, 

 a little hardier, a little less readily preceived by their 

 enemies, than the others. They are the winners in the 

 struggle for existence; they are the survivors. And this 

 survival of the fittest, as it is called, is practically a process 

 of selection by Nature. Nature selects the fittest to live 

 and to perpetuate the species. Their progeny again 



