THE FACTORS. 395 



pons ; lastly by their intelligence, without which, indeed, 

 their other superiorities would avail them little. And then 

 these various active powers serving for defence, become, in 

 other cases, the powers that enable animals to aggress, and to 

 preserve their lives by the success of their aggressions. 



The second process by which extinction is prevented the 

 formation of new individuals to replace the individuals 

 destroyed is carried on, as described in the chapter on 

 " Genesis," by two methods, the sexual and the asexual. 

 Plants multiply by spontaneous fission, by gemmation, by 

 proliferation, and by the evolution of young ones from de- 

 tached cells and scales and leaves ; and they also multiply 

 by the casting off of spores and sporangia and seeds. In like 

 manner among animals, there are varied kinds of agamo- 

 genesis, from spontaneous fission up to parthenogenesis, all of 

 them conducing to rapid increase of numbers ; and we have 

 the more familiar process of gamogenesis, also carried on 

 in a great variety of ways. This formation of 



new individuals to replace the old, is, however, inadequately 

 conceived if we contemplate only the number born or detached 

 on each occasion. There are four factors, all variable, on 

 which the rate of multiplication depends. The first is the 

 age at which reproduction commences ; the second is the 

 frequency with which broods are produced ; the third is the 

 number contained in each brood ; and the fourth is the length 

 of time during which the bringing forth of broods con- 

 tinues. There must be taken into account a further element 

 the amount of aid given by the parent to each germ in the 

 shape of stored-up nutriment, continuous feeding, warmth, 

 protection, &c. : on which amount of aid, varying between 

 immensely wide limits, depends the number of the new indi- 

 viduals 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 



