GENESIS. 289 



Those kinds of animals which multiply by heterogenesis, 

 present us with a parallel relation between the recurrence of 

 gamogenesis and the recurrence of conditions checking rapid 

 growth: at least, this is shown where experiments have 

 thrown light on the connexion of cause and effect ; namely, 

 among the Aphides. These creatures, hatched from eggs in 

 the spring, multiply by agamogenesis, which in this case is 

 parthenogenesis, throughout the summer. When the weather 

 becomes cold and plants no longer afford abundant sap, per- 

 fect males and females are produced; and from gamogenesis 

 result fertilized ova. But beyond this evidence we have 

 much more conclusive evidence. For it has been shown, 

 both that the rapidity of the agamogenesis is proportionate 

 to the warmth and nutrition, and that if the temperature and 

 supply of food be artificially maintained, the agamogenesis 

 continues through the winter. Nay more — it not only, under 

 these conditions, continues through one winter, but it has 

 been known to continue for four successive years: some 

 forty or fifty sexless generations being thus produced. And 

 those who have investigated the matter see no reason to 

 doubt the indefinite continuance of this agamogenetic mul- 

 tiplication, so long as the external requirements are duly 

 met. Evidence of another kind, complicated by 



a net when these are above a certain size and age; and then only under 

 conditions unfavourable to growth, such as a feeble light or poverty of 

 nutritive inorganic salts or absence of oxygen, or a low temperature in the 

 water containing the plant. The presence of organic substances, especially 

 sugar, also acts as a stimulus to the formation of gametes, and this is also 

 the case in Vavchcria. Many other Alr^ce produce gametes mainly at the 

 end of the vegetative season, when food is certainly difficult to obtain in their 

 natural habitat, and we may well suppose that their assimilative power is 

 waning. Where, however, as is the case in Vaucheria, the plant depends for 

 propagation mainly on the production of fertilized eggs, we find the sexual 

 organs often produced in conditions very favourable to vegetative growth, 

 in opposition to those cases such as Hydrodidyon^ where the chief means 

 of propagation is by zoospores. So that side by side with, and to some 

 extent obscuring, the principle developed above we have a clear adaptation of 

 thd production of reproductive cells to the special circumstances of the case," 



