THE FACTORS. 415 



crevices, or by forming burrows and nests. They save them- 

 selves from enemies by developed powers of locomotion, 

 taking the shape of swiftness or agility or aptitude for 

 changing their media; by their strength either alone or 

 aided by weapons; 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 ani- 

 mals 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 con- 

 ceived 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 

 continues. 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 feed- 

 ing, warmth, protection, &c. : on which amount of aid, vary- 

 ing between immensely wide limits, depends the number of 



