THE ORIGIN OF ANIMAL SPECIES 453 



hundreds, and fishes millions, of eggs. If all the young 

 were to survive, the earth would soon be unable to sup- 

 ply sufficient food and standing-room. 



(2) In spite of this tendency to increase inordinately, the 

 number of animals remains, on the whole, stationary. 

 Even though there may be an enormous temporary in- 

 crease in the number of certain animals, as in " plagues 

 of grasshoppers," normal conditions are soon restored by 

 natural agencies. Eggs and young are devoured by older 

 animals. Disease, old age, parasites, enemies, storms, 

 floods, cold, heat, drought, and famine are responsible 

 for the death of so many individuals that comparatively 

 few young animals of any species live to maturity. 



(3) There results, consequently, severe competition for 

 the necessaries of life, a veritable struggle for existence. 

 In order to thrive, animals need food, shelter from the 

 elements, protection from enemies, and freedom from 

 molestation while rearing their young. Deprivation of 

 any of these is likely to be followed by serious results for 

 the animals as individuals and for the race as a whole. 

 The introduction of sheep has made it impossible for 

 cattle to live on some of the Western ranges, because the 

 sheep crop the grass so closely that there is not enough 

 left to feed the cattle. The " English," or house, spar- 

 row appropriates the best protected nesting places, raises 

 several broods each season, eats whatever food is avail- 

 able, and remains the year round without migrating. By 

 reason of these habits it has been victorious in the con- 

 test for the places formerly occupied by native birds. 

 The struggle for existence is most keen between closely 

 related forms, since each will naturally want what the 

 other desires. Until about two centuries ago the black 

 rat was the common rat of Europe. Since then it has 

 been driven out by the brown rat, a larger and stronger 

 species. 



