THE STRUGGLE FOR EXISTENCE 405 



Some animals produce vast numbers of eggs or young; 

 for example, the herring, 20,000; a certain eel, several 

 millions; and the oyster from 500,000 to 16,000,000. 

 Supposing we start with one oyster and let it produce one 

 million of eggs. Let each egg produce an oyster which 

 in turn produces * one million of eggs, and let these go 

 on increasing at the same rate. In the second generation 

 there would be one million million of oysters, and in the 

 fourth, i.e. the great great grandchildren of the first oyster, 

 there would be one million million million million of 

 oysters. The shells of these oysters would just about 

 make a mass the size of the earth. 



But it is obvious that all the new individuals of any 

 animal produced do not live their normal duration of life. 

 All animals produce far more young than can survive. 

 As a matter of fact, which we may verify by observation, 

 the number of individuals of animals in a state of nature is, 

 in general, about stationary. 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. 

 Some species increase in numbers, as for example, the 

 rabbit in Australia, which was introduced there in 1860 

 and in fifteen years had become so abundant as to be a 

 great pest. Other species decrease, as the buffaloes, which 

 once roamed our great plains in enormous herds and are 

 now represented by a total of a few hundred individuals, 

 and the passenger-pigeon, whose migrating flocks ten years 

 ago darkened the air for hours in parts of the Mississippi 

 valley, where now it is a rare bird. But the hand of man 

 is the agent which has helped to increase or to check the 

 multiplication of these animals. In nature such quick 

 changes rarely occur. 



* Oysters are hermaphroditic, each individual producing both sperm- and 

 egg-cells. 



