148 THE LAW OF BIRTHS AND DEATHS 



usually such as " enjoy from time to time very favourable 

 conditions, which they can make use of for very prolific 

 reproduction, and then pass into very dangerous and 

 unfavourable conditions, which they survive in the shape 

 of fertilized and hard-shelled eggs." l 



Unicellular organisms are not only the source from 

 which the higher forms of life are derived, but, together 

 with the lower forms of multicellular organisms, they 

 form the ultimate source of the food supply of the higher 

 species. In order that this may be it is necessary that 

 they should be able to multiply with extraordinary rapidity 

 when circumstances are favourable. This is rendered 

 possible by the asexual methods of reproduction. 



Sexual reproduction is essentially a regulating factor. 

 It becomes the exclusive method of reproduction among 

 the higher animals among whom unregulated multiplica- 

 tion would be disastrous, and among the lower forms of 

 life it appears when conditions become unfavourable to 

 very rapid multiplication. Aphides or plant lice provide 

 an excellent illustration of the relations between the 

 two forms of reproduction. " Details in regard to these 

 plant lice which multiply so rapidly upon our rose-bushes, 

 fruit-trees and the like, differ somewhat in the various 

 species, but the general facts are recognised to be as 

 follows : During the summer months the aphides produce 

 parthenogenetically generation after generation of females. 

 The advent of autumn, however, with its attendant cold 

 and scarcity of food, brings about the birth of males 

 and the consequent recurrence of strictly sexual repro- 

 duction. In the artificial environment of a greenhouse, 

 equivalent to a perpetual summer of warmth and abundant 

 food, the parthenogenetic succession of females has been 

 experimentally observed for four years ; it seems, in fact, 

 to continue until lowering of the temperature and diminu- 

 1 Darwinism and the Problems of Lije, Guenther. 



