GENERAL LAW OP ANIMAL FERTILITY. 581 



destroyed. On the other hand, partly bv the strength, swiftness 

 and sagacity of its members, and partly by their fertility, it is 

 constantly being maintained. These conflicting sets of influences 

 may be conveniently generalized as the forces destructive of 

 race, and the forces preservative of race. 



2. Whilst any race continues to exist, the forces destructive 

 of it and the forces preservative of it must perpetually tend 

 towards equilibrium. If the forces destructive of it decrease, 

 the race must gradually become more numerous, until, either 

 from lack of food or from increase of enemies, the destroying 

 forces again balance the preserving forces. If, reversely, the 

 forces destructive of it increase, then the race must diminish, 

 until, either from its food becoming relatively more abundant, 

 or from its enemies dying of hunger, the destroying forces sink 

 to the level of the preserving forces. Should the destroying 

 forces be of a kind that cannot be thus met (as great change of 

 climate), the race, by becoming extinct, is removed out of the 

 category. Hence this is necessarily the law of maintenance of all 

 races ; seeing that when they cease to conform to it they cease to be. 



Now the forces preservative of race are two ability in each 

 member of the race to preserve itself, and ability to produce 

 other members power to maintain individual life, and power to 

 propagate the species. These must vary inversely. When, from 

 lowness of organization, the ability to contend with external 

 dangers is small, there must be great fertility to compensate 

 for the consequent mortality ; otherwise the race must die out. 

 When, on the contrary, high endowments give much capacity of 

 self-preservation, there needs a correspondingly low degree of 

 fertility. Given the dangers to be met as a constant quantity ; 

 then, as the ability of any species to meet them must be a con- 

 stant quantity too, and as this is made up of the two factors 

 power to maintain individual life and power to multiply these 

 cannot do other than vary inversely. 



3. To show that observed phenomena harmonise with this 

 a priori principle seems scarcely needfuL But, though axiomatic 

 in its character, and therefore incapable of being rendered more 

 certain, yet illustrations of the conformity to it which nature 

 everywhere exhibits, will facilitate the general apprehension of it. 



In the vegetable kingdom we find that the species consisting 

 of simple cells, exhibit the highest reproductive power. The yeast 

 fungus, which in a few hours propagates itself throughout a large 

 mass of wort, offers a familiar example of the extreme rapidity 

 with which these lowly organisms multiply. In the Protocofc** 

 nivalis, a microscopic plant which in the course of a night reddens 



