HETEROGONY OF THE DAPHNIDS 115 



on the view of an occasional cross being indispensable."^ Sub- 

 sequent searches have shown, however, that parasitic worms, 

 when they exist alone in the body of a host, are capable of self- 

 fertilisation, and this faculty may be interpreted as a theoretical 

 as well as a practical necessity in regard to the persistence of 

 the species. But this occasional self-fertihsation does not 

 exclude the possibility of crossing, and these parasites are also 

 capable of the latter mode of reproduction. 



With regard to the " indispensable " nature of crossing, on 

 which Darwin has insisted, the remarkable cases of heterogony, 

 or regular succession of parthenogenesis and sexual reproduction, 

 observed in the Daphnids seem to prove once more that amphi- 

 mixis is not so much an indispensable condition of the continu- 

 ance of life as an adaptation, exceedingly effective in certain 

 circumstances, which secures the stabiUty and progression of 

 the species. The conditions of life of the Daphnids are favour- 

 able only for a short period, wMle during a much longer period 

 they are distinctly unfavourable. The habitats of these crus- 

 taceans — ditches, ponds, bogs — are frequently frozen in winter 

 and dried up in summer, and the conservation of the species can 

 only be effected by the production of eggs capable of with- 

 standing frost or drought. These eggs are always female, and 

 their product invariably reproduces by parthenogenesis, so that 

 a large number of offspring can be rapidly produced when the 

 conditions permit. During the period in which the conditions 

 are favourable — that is to say, in which the habitat is open — a 

 generation of males is produced, and the thick-shelled winter or 

 summer eggs are the result of the previous fertilisation of the 

 females. Thus we have a clear illustration of the fact that the 

 mode of reproduction is adapted to the conditions of life. If the 

 individuals arising from the winter or summer eggs had first of 

 all to seek a sexually differentiated partner before reproducing 

 themselves, it is obvious that the chances of the survival of the 



1 Darwin, The Origin of Species, p. 123. 



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