REPiCODUCTION 



25 



Lastly, in some minute aquatic 

 Crusters and many Rotifers (water- 

 worms) no males have ever been 

 found. Propagation must, therefore, 

 take place altogether without males, 

 thus giving us the phenomenon of 

 total Parthenogenesis. 



Seeing that parthenogenetic repro- 

 duction is by no means rare, and 

 that the number of such offspring is 

 abundant, it becomes once more 

 evident from these facts that sexual 

 union is by no means essential for 

 the propagation of life. 



(d) Alternation of Generations. 



We have just described the occur- 

 rence of parthenogenetic females in 

 plant-lice during summer which 

 alternate with the sexual generation 

 of males and females of the autumn. 

 As the summer breeds differ from 

 the autumn breeds, we have here a 

 true case of alternation of genera- 

 tions. In the cited instance sexual 

 generations alternate with partheno- 

 genetic ones. But the succession 

 may be between sexual and asexual 

 generations, the latter propagating 

 by division, budding, or sporulation. 



The former case is illustrated by 

 the Hydra-polypes and the common 

 jelly-fish (Aurelia). In both these 

 instances we find free - swimming 

 sexual individuals producing a sessile 

 generation, which is asexual, and 



Fig. 18. — Alternation 

 OF Generations in 

 Plant-Lice. 



{From Geddes and 

 Thomson, " The Evo- 

 lution of Sex.") 



At the base is an indi- 

 vidual arising from 

 a fertilized egg-cell ; 

 this gives origin par- 

 thenogenetically to a 

 succession of genera- 

 tions. At the top are 

 shown the male and 

 female forms, which 

 ultimately reappear. 

 At the side an earlier 

 reappearance of 

 sexual forms is sug- 

 gested. 



