4 \ Cutler Parthenogenesis in Animals 



External Factors as the Cause. of Parthenogenesis 



A great deal of work hae been done in trying to ascertain 

 whether or not the (external conditions play the largest part in 

 causing the change from the parthenogenetic mode of reproduc- 

 tion to that of £he sexual one. Kurz in 1874 showed that if the 

 water in which Daphnia were living was slowly evaporated 

 sexual forms appeared, and it was suggested that the increased 

 concentration of the salts [was responsible for the change. In 

 1905 a paper appeared by Issakowitsch, showing that starvation 

 and low temperature were wholly responsible for the changed 

 mode of reproduction. The view that external conditions were 

 not the sole agents |n the production of this change was advo- 

 cated by Weissman in /1875. His conclusion was that the animals 

 were so constituted by natural selection that they tend spon- 

 taneously to reproduce sexually in the appropriate season ; and 

 that they so do to a large degree irrespective of the external 

 conditions. Thus, according to this observer, the change from 

 parthenogenesis to sexual reproduction is an inherited character. 

 A more recent worker takes an intermediate view; thus, accord- 

 ing to Papanicolau, the external and internal conditions act 

 together in the production of males and sexual females. He. 

 recognises three periods : 



1. Purely parthenogenetic period comprising the first few 

 generations. 



2. Transition period, when warmth induces parthenogenesis 

 and cold sexual reproduction. 



3. Late period, when the females are sexual and no external 

 conditions can cause them to become parthenogenetic. 



Agar, however, does not entirely agree with the above con- 

 clusions. From work done on Simoeephalus vetulus, he says 

 that there is no justification for stating that sexual forms appear 

 after a certain number of generations have elapsed since the 

 last fertilised egg ; and that the onset of sexuality is influenced 

 by environmental factors. It has long been known that among 

 the Aphids during the Summer months reproduction is entirely 

 parthenogenetic, and it was generally assumed that the cold of 

 the later months caused the appearance of sexual forms. Kyber 

 in 1 85 1 experimented on the rose aphid (Aphis rosse). By keeping 

 the animals in a warm chamber he was able to extend the par- 

 thenogenetic cycle for four years. Identical results were also 

 obtained by Slingerland in 1893. Comparatively little experi- 

 mental work has been done in this group, but it is clear that 

 external agents do have 'some influence on the life cycle of some 

 species of Aphids. 



