416 LAWS OF MULTIPLICATION. 



the new individuals that survive long enough to replace the 

 old, and perform the same reproductive process. 



Thus, regarding every living organism as having a moving 

 equilibrium dependent on environing forces, but ever liable 

 to be overthrown by irregularities in those forces, and always 

 so overthrown sooner or later; we see that each species of 

 organism can be maintained only by the generation of new 

 individuals with a certain rapidity, and by helping them 

 more or less fully to establish their moving equilibria. 



318. Such are the factors with which we are here con- 

 cerned. I have presented them in abstract shapes for the 

 purpose of showing how they are expressible in general 

 terms of force how they stand related to the ultimate laws 

 of redistribution of matter and motion. 



For the purposes of the argument now to follow, we may, 

 however, conveniently deal with these factors under a more 

 familiar guise. Ignoring their other aspects, we may class 

 the factors which affect each race of organisms as forming 

 two conflicting sets. On the one hand, by what we call 

 natural death, by enemies, by lack of food, by atmospheric 

 changes, &c., the race is constantly being destroyed. On the 

 other hand, partly by the endurance, the strength, the swift- 

 ness, and the sagacity of its members, and partly by their 

 fertility, it is constantly being maintained. These conflicting 

 sets of factors may be generalized as the forces destructive 

 of race and the forces preservative of race. So generalizing 

 them, let us ask what are the necessary implications. 



