CHANGES IN THE COMMUNITY 27 



This limitation is imposed by the process of sexual 

 generation, under which new individuals are 

 created by the combination of two parents, and, 

 owing their existence to two parents, are exact 

 copies of neither of them. It is, of course, quite con- 

 ceivable that a single individual should of itself 

 have the power of producing offspring, and in the 

 lowest classes of the animal and vegetable king- 

 doms we find that single organisms produce new 

 organisms simply by dividing themselves, or by 

 throwing out buds. Here, then, we have reproduc- 

 tion without sex, which may act, moreover, with 

 most extraordinary rapidity, the descendants of a 

 single cell multiplying themselves into several 

 millions within a few hours. But this process 

 will not generally continue indefinitely. After a 

 time the swarm appears to lose vitality, and can 

 no longer increase by the division of single cells. 

 Strength is recovered by the process of " conjuga- 

 tion " : two cells come together and blend their 

 substance into a single mass, which subsequently 

 breaks up into a fresh swarm of offspring. It is 

 believed that the two conjugating cells have, 

 before their union, developed the sexual characters 

 which, in less primitive organisms, distinguish 

 the " sperm-cell " of the male from the " germ- 

 cell " of the female. 



In all but the very simplest organisms, 

 throughout both the vegetable and the animal 

 kingdoms, special organs of two kinds are 

 developed for the production of these cells. 

 Both kinds may be borne by the same individual, 

 in which case, of course, no individual distinction 

 of sex arises. Amongst plants, the sperm-cells 

 are produced by the anthers of the stamens : the 

 germ-cells by the ovary. These, again, are some- 

 times borne by different flowers sometimes on 

 different plants but they usually form parts 



