124 



THE I5FLUEXCE OF INAXIiMATE SHRItOUNDINGS. 



thenogeiiesis were not exposed to the effect of cold, but kept in 

 a constant summer heat, and at the same time supplied with 

 suitable food, no males would occur, and the young would be 

 uninterruptedly produced by the parthenogenetic mode. In 

 fact Reaumur did succeed in this way in producing artificially 

 above fifty parthenogenetic generations in the course of three or 

 four years, all descended from one mother. The converse ex- 

 periment has never, so far as I know, been made — namely, 

 whether it would be possible to produce males in the spring by 





Fig. 31.— a, Diplozoon paradoxum, from tlie gill of a fresti-water fish ; &, Polystomum 

 in(egeTnmum, from the bladder of a frog. 



artificially lowering the temperature, although they do not pro- 

 perly appear till later in the year, and thus to diminish the 

 normal number of .'■uccessive parthenogenetic generations in a 

 summer. It is highly probable that it might succeed. 



The facts lately communicated by Zeller are no less interest- 

 ing. He found that certain parasites, in precise opposition to 

 the Aphides we have been speaking of, JJipIozoon paradoxum 

 and Polystomum iniez/frrhnnm (fig. 31), produce true eggs in 

 the summer only, which must be fertilised before they can 



