21 8 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



such species as Cyprois monacha, of which every generation 

 reproduces sexuall}^ ; the second was found in those species in 

 which numerous parthenogenetic generations alternate with 

 a sexual one ; and finally the third included species in which 

 males have not 3^et been found: in one such species {Cypris 

 reptmis), forty consecutive purely parthenogenetic generations 

 have been observed. 



We cannot yet decide why the advantages of amphimixis 

 have been entirely given up in this and other cases. We can- 

 not at present solve, or even profitably discuss, every biological 

 problem. But it is probable that we are dealing not with 

 adaptation alone, but perhaps with a suppression of amphi- 

 mixis by means of parthenogenesis. Everything which is 

 desirable is not possible ; and after parthenogenesis has once 

 been incorporated in the hereditary tendencies of a species, 

 circumstances may arise in consequence of which it may be 

 transferred, by the power of heredity, to the remaining sexual 

 generations also, without the possibility of any interference 

 on the part of natural selection. Whether this explanation 

 is in the right direction or not, it is at any rate clear, as 

 regards the question under discussion (viz. the true significance 

 of amphimixis), that the loss of an advantage may be intel- 

 ligible in many ways, while the loss of a process of vital reju- 

 venescence must stand in direct opposition to the continuance 

 of life. 



It would be of the highest interest to consider more closely 

 the various cases of parthenogenesis, from this point of view : 

 we do not, however, possess sufficiently accurate knowledge of 

 the vital relations of the animals in question to enable us to 

 estimate the advantages conferred by the disappearance of 

 amphimixis, or rather the introduction of parthenogenesis, in 

 a larger or smaller number of generations. I may, nevertheless, 

 be permitted to afford some indication of the line of argument. 



Parthenogenesis plays an important part in the group of plant- 

 lice and bark-lice, containing very numerous species. The ova 

 may be deposited or may undergo embryonic development 

 within the bod}^ of the mother. In either case the advantage 

 of parthenogenesis depends, as in the Daphnids, on the ex- 

 traordinary rate of multiplication, which naturally reaches the 

 highest point in the viviparous Aphidae, because the offspring 



