4-6 THE EVOLUTION OF SEX. 



somewhat in the various species, but the general facts are re- 

 cognised to be as follows. During the summer months, with 

 favourable temperature and abundant food, the aphides produce 

 parthenogenetically generation after generation of females. The 

 advent of autumn, however, with its attendant cold and scarcity 

 of food, brings about the birth of males, and the consequent 

 recun-ence of strictly sexual reproduction. In the artificial 

 environment of a greenhouse, equivalent to a perpetual summer 

 of warmth and abundant food, the parthenogenetic succession 

 of females has been experimentally observed for four years, — it 

 seems in fact to continue until lowering of the temperature and 

 diminution of the food at once re-introduce males and sexual 

 reproduction. 



(e) Butterflies and Moths. — Still keeping to insects, we may 

 note Mrs Treat's interesting experiment, that if caterpillars were 

 shut up and starved before entering the chrysaHs state the 

 resultant butterflies or moths were males, while others of the 

 same brood highly nourished came out females. Gentry too 

 has shown for moths, that innutritious or diseased food produced 

 males, and suggests this as a partial explanation of the excess 

 of male insects in autumn, although we suspect that tempera- 

 ture is in this instance probably more important. 



(/) Crustaceans. — In support of the same contention, Rolph 

 has drawn attention to the following among other facts. One 

 of the brine shrimps {Arteniia salind) resembles not a few 

 crustaceans in the local and periodic scarcity or absence of 

 males, associated of course with parthenogenesis. At Mar- 

 seilles, Rolph says, this artemia lives in especially favourable 

 conditions, as its large size plainly indicates ; there it produces 

 only females. '\Miere the conditions of existence are less 

 prosperous, it produces males as well. " A certain maximum 

 of abundance and optimum of vital conditions in partheno- 

 genetic animals — daphnids and aphides, Apus, Branchipus, 

 Artemia, and numerous other crustaceans — produce females ; 

 while less favourable conditions are associated with the produc- 

 tion of males." In regard, however, to water-fleas (daphnids), it 

 is fair to notice that Rolph's conclusions do not quite consist 

 with Weismann's, who, with unique experience in regard to 

 these curious little animals, is disinclined to allow the direct 

 influence of temperature and nutrition in the matter, 



ig) Mamjnals. — When we pass to higher animals, the diffi- 

 culties of pro^"ing the influence of nutrition upon sex are much 



