400 THE ANIMALS AND MAN 



two eggs yearly, that is, each pair yearly multiplies, not 

 simply replaces itself. Most birds, however, are slow multi- 

 pliers. But what of the hosts of insects where each female 

 lays from a few dozen to many hundred or even thousand 

 eggs each year; and the fishes, almost none of which lays 

 less than several thousand a year? A few years of unin- 

 terrupted normal increase among sunfishes would fill every 

 stream and pond solidly full of them. Even certain of the 

 tiniest animals, microscopic animalcules which live in the 

 ocean, if left to multiply at their usual rate with no losses 

 except by natural death, would, it has been estimated, 

 completely fill the ocean in about a week! 



Of course no such appalling increase in the number of 

 living animals occurs, although we may fairly consider 

 that each kind of animal is constantly trying to usurp far 

 more food and space in the world than it now has. But 

 there are about as many squirrels in the forest one year as 

 another, about as many butterflies in the field, about as many 

 frogs in the pond. Sometimes a particular kind of ani- 

 mal gets into a new part of the world and suddenly mul- 

 tiplies with great rapidity. A few rabbits were introduced 

 into Australia (where there were none) in i860, and in fifteen 

 years had become so abundant as to be a great pest. The 

 government pays large sums in bounties every year to rab- 

 bit-hunters. 



The struggle to live. — All animals tend to increase in 

 geometrical ratio, that is, the production of new individuals 

 is by multiplication, not by simple addition. But food and 

 space on the earth have definite limits, and so there is con- 

 stantly going on a great struggle for existence. In the case 

 of any individual the struggle is threefold: (i) with the 

 other animals of his own kind or species for food and room; 



(2) with other kinds of animals which want the same food 

 and space, or which may want him for food; and finally, 



(3) with the conditions of life, such as cold and heat, and 



