THE FERTILITY OF NATURE. 107 



there would be no room in the seas and oceans for 

 the masses of fishes which would result from the 

 fertility of Nature having full swing. 



In lower life, as well, the same fertility is to be seen, 

 contrasting markedly with the very limited rate of 

 production witnessed in some animal species, whereof 

 the elephant is a good example. A tapeworm, as an 

 internal parasite, may, and does as a rule, consist of 

 several hundreds of joints. Each joint contains several 

 thousands of eggs, and the number of young tape- 

 worm guests which would seem to be capable of being 

 launched on a world of inoffensive hosts at first sight 

 appears out of all reasonable proportion. Yet, as 

 science teaches us, the chances of a tapeworm-egg 

 ever reaching maturity of development must be ranked 

 by many thousands to one. 



Our parasite's egg has to pass through such a com- 

 plex cycle of development, and its chances of destruc- 

 tion are so many and varied, that we need not wonder 

 that while tapeworm-eggs are many, the adult parasites 

 are so few. Or take the case of the plant-lice, or 

 " green flies " (Aphides), that swarm on our plants in 

 summer. These insects, as the gardener knows to his 

 cost, are innumerable. They exist in myriads, it is 

 true, but even their fertility is checked by cold, and by 

 the many enemies that prey upon them. Thus, although 

 the fertility of Nature is unbounded in one sense, it is 

 checked in another. There is a prolific progeny as 

 a rule ; but it is prolific, first of all, because this is 

 Nature's own way of encouraging, by sheer force of 

 numbers, the production of animals and plants, out 

 of whose ranks new and better individuals will step 

 to advance the race and promote the change that 

 makes for progress. 



