12 THE PHYSIOLOGY OF REPRODUCTION 



is dependent upon temperature. In a favourable summer the 

 females of this animal may produce as many as fourteen con- 

 secutive generations of young by parthenogenesis, the ova 

 undergoing development without being fertilised by the male. 

 At the beginning of the winter male plant-lice make their 

 appearance and fertilise the eggs, which develop in the succeeding 

 spring. Reaumur, however, by artificially maintaining a con- 

 stant summer temperature, succeeded in producing more than 

 fifty parthenogenetic generations of plant-lice, all descended 

 from a single mother. 1 



Morgan, however, describes some observations which seem 

 to indicate that the change is not merely due to temperature. 

 He shows, for example, that the sexual forms of Aphis may 

 appear in the autumn before the onset of the cold weather, and 

 conversely that many individuals may continue to reproduce 

 parthenogenetically, until finally they perish from the cold. 

 Morgan suggests that the alternation in the mode of reproduction 

 may depend upon changes which take place in the food-plant 

 in the autumn, instead of being solely a temperature effect. 

 He shows also that there is evidence for the conclusion that in 

 the genus Chermes, in which the alternation of generations 

 occurs between the fir-tree and the larch, the conditions existing 

 on the larch are those that call forth the sexual forms. 2 



It has been supposed that the change in the environment is 

 also responsible for determining the sexes in aphids. Miss 

 Stevens, however, has recently shown that what appears to be a 

 change in sex should rather be regarded as a change from the 

 parthenogenetic to the sexual mode of reproduction. 3 According 

 to this view the sex of each individual is determined by the 

 character of the gamete or gametes by which it is developed. 

 The supposed influence of food and external conditions upon 

 sex-determination in various kinds of insects, and other animals, 

 is discussed at some length in a future chapter of this work 

 (Chapter XV.). 



Semper pointed out long ago that the occurrence of repro- 

 duction (or of the particular mode of reproduction), with 



1 Semper, loc. cit. z Morgan, loc. cit. 



3 Stevens, " Studies in the Germ-Cells of Aphids," Carnegie Institution 

 Report, Washington, 1906. 



