4 o THE EVOLUTION OF SEX. 



that in parthenogenesis, where the production of males is the normal 

 condition, favorable environmental influences appear to introduce 

 females. 



Fig. 13. — Two forms of a common Plant-louse or Aphis. 



This figure equally well illustrates three different things: 

 a winged male and a -wingless female: a. winged and a 

 wingless parthenogenetic female; a winged sexual female; 

 and an ordinary parthenogenetic female. — From Kessler. 



id) The Case of Aphides. — One of the most familiar illustrations 

 of the influence of nutrition upon sex is found in the history of the 

 plant-lice or aphides, which is indeed full of other suggestions in 

 regard to the whole theory of sex and reproduction. Details in 

 regard to these plant-lice, which multiply so rapidly upon our rose- 

 bushes, fruit-trees, and the like, differ somewhat in the various species, 

 but the general facts are recognized to be as follows. During the sum- 

 mer months, with favorable 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 recur- 

 rence 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 

 reintroduce 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 chrysalis 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 expla- 

 nation of the excess of male insects in autumn, although we suspect 

 that temperature is in this instance probably more important. 



