220 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



to be saved, but also for every survivor in the struggle to be 

 capable of lajang eggs with the power of developing unaided, 

 in other words for every such animal to be a female. 



Much might still be said as to the causes of the omission of 

 amphimixis from one or more generations, but a few words 

 will suffice to show that the appearance of parthenogenesis 

 depends upon adaptation to the conditions of life, — that reproduc- 

 tion without amphimixis has invariably originated from sexual 

 reproduction, whenever it zuas i^equired in order to gain some 

 distinct advantage in the effort to maintain the species. We may 

 well assume that the advantages which the appearance of 

 parthenogenesis must confer, outweigh the disadvantages in- 

 volved in the giving up of amphimixis. Our estimate as to 

 the effects of the latter is far less certain and precise than of 

 the former. If, however, I am not mistaken in my views on 

 the significance of amphimixis as the source of individual varia- 

 tion, it follows that its omission from a single generation or 

 even from a series of generations may be easily compensated ; 

 for it alwa3^s reappears, and mingles afresh the complex indi- 

 vidual predispositions into new combinations. The injury 

 caused b}' its withdrawal would be less as the fertility of 

 the species was greater ; with this is connected the fact that 

 parthenogenesis is chiefly found in very prolific species. Those 

 individuals which sink below the level of organization charac- 

 teristic of the species could the more easity be eliminated in the 

 struggle for existence without in any way endangering the life 

 of the species. Perhaps this explains why, in some few species 

 of Crustacea {Cypris) and of Insecta (Rhodites rosae), amphimixis 

 has utterl}'' vanished without having caused, up to the present 

 time, any trace of degeneration in the species. 



We may safely assume that the entire absence of amphi- 

 mixis is to be primarily explained as an adaptation, and that 

 the alternation between sexual and asexual multiplication 

 met w^ith in H3'dromedusae, Cestoda, &c., has arisen from 

 the demands made bj'^ the conditions of life, — demands similar 

 to those which have determined the alternation between mono- 

 sexual and bisexual generations found in Insecta, Crustacea, 

 &c. In both classes of cases amphimixis has been restricted 

 to certain generations because it was not necessary in all of 

 them, and because such restriction w^as a great advantage. 



