HERMAPHRODITISM 113 



•whicli ampiimixis is excluded can maintain itself as long as any 

 members of tliat species survive, for these wiU go on begetting 

 progeny wbicb are free from inherited degeneracy. 



It is very difficult to say with any degree of certainty how 

 long the species which we know as parthenogenetic have been 

 in this condition. Weismann reared eighty successive genera- 

 tions of Cyprus reptams without producing a single male indi- 

 vidual, which seems to show that the view which interprets 

 amphimixis as a necessary " rejuvenating " factor is incorrect. 

 On the other hand, in other cases of parthenogenetic species, a 

 male occurs from time to time, which would seem to show that 

 the institution of parthenogenesis is not in these cases of very 

 ancient origin. The persistence of the receptacidum seminis in the 

 females of some parthenogenetic species shows that after more than 

 one himdred generations have elapsed since the functional use 

 of an organ, this organ nevertheless remains. This proves that 

 the intragenninal variations which lead to the regression of use- 

 less organs are purely fortuitous ; but when such an accidentally 

 produced regressive variation of an organ which is biologically 

 indifierent to the species sets in, natural selection does not set 

 any obstacle in the way of its further progress. 



Turning now to those species which habitually reproduce 

 themselves by consanguineous mating, we find very close con- 

 sanguinity prevaUing among certain species Hving in colonies — 

 for instance, among certain ants, where the male and female of 

 each pair always belong to the same colony. We find herma- 

 phroditism chiefly in species which have lost their power of 

 movement, and which are rooted to one spot, as in the case of 

 oysters, barnacles, various corals, etc. Hermaphroditism is 

 found also in all the species of terrestrial snails, and both self- 

 fertihsation and cross-fertilisation may occur, according to cir- 

 cumstances, among certain species, such as the Cirripedes. 



That the mode of reproduction, as the most important factor 

 in the life of the species, is always adapted to circumstances is 



