2 74 FIRST LESSONS IN ZOOLOGY 



tiplies, not simply replaces itself. Most birds, however, 

 are slow multipliers. But what of the host of insects 

 where each female lays from a few dozen to many hun- 

 dred or even thousand eggs each year; and the fishes, 

 almost none of which lays less than several thousand a 

 year ? A few years of uninterrupted normal increase 

 among sunfishes would fill every stream and pond solidly 

 full of them. Even certain of the tiniest animals, micro- 

 scopic animalcules which live in the ocean, if left to mul- 

 tiply 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 \\orld 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 rabbit-hunters. 



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

 geometrical ratio, that is, the production of new indi- 

 viduals is by multiplication, not by simple addition. 

 But food and space on the earth have definite limits, and 

 so there is constantly going on a great struggle for exist- 

 ence. 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 Avhich want the same food and space, or M-hich 

 may \vant him for food; and finally, (3) with the condi- 



