INTRODUCTION 29 



bowels of which it develops into a young animal. 

 This passes into the muscles, and buries itself therein. 

 Then the cow must be killed, and its flesh be eaten 

 by a human being, for it is only in the human intestines 

 that the tiny creature will grow into the adult tape- 

 worm. We can easily see that the tape-worm would 

 soon become extinct if it were less fruitful. 



We might go through the whole series of animals, 

 and we should find in every species a confirmation of 

 the fact that every animal's fertility is proportionate 

 to its perils. We may add that it is also proportionate 

 to the food and space provided for it, since it is clear 

 that, if foxes multiplied as mice do, they would soon 

 consume all the animal food in their environment, and 

 would be doomed themselves ; whereas vegetarian 

 animals, for instance, would find a far more abundant 

 diet. It is also obvious that animals with a limited 

 habitat must have a low degree of fertility, otherwise 

 they deprive themselves of food, light, and space, and 

 court destruction. 



We shall explain at a later stage the fact that each 

 species produces, on the average, just as many young 

 as is necessary for its maintenance, and that, therefore, 

 its production increases in proportion to the dangers 

 it encounters. Here we need only observe that every 

 animal has in itself the power of multiplying in- 

 definitely. Thus is brought about the struggle for life, 

 the unceasing fight for food, space, and light. 



This struggle only affects the number of organisms. 

 It restrains each species within the limits that are set 

 to its expansion. But there is another struggle for 



