184 HEREDITY AND SEX 



In this life history we do not know what changes 

 take place in the chromosomes. It has, however, often 

 been claimed in this case that the transition from par- 

 thenogenesis to sexual reproduction is due to changes in 

 the environment. 



In fact, this is one of the stock cases cited in the older 

 literature to show that sex is determined by external 

 agents. It was said, that if the environment causes 

 males to appear, then sex is determined by the environ- 

 ment. But as a matter of fact, in so far as changes in 

 the environment affect this animal, they cause it to 

 cease reproducing by parthenogenesis, and induce sexual 

 reproduction instead. The evidence is consistent in 

 showing that any external change that affects the 

 mode of reproduction at all calls forth either sexual 

 eggs or males. The machinery of parthenogenesis 

 is switched off, and that for sexual reproduction is 

 turned on. 



The discrepancies that appear in the older accounts 

 are probably due, as Papanicolau has shown, to dif- 

 ferent observers using females that belong to different 

 phases of the parthenogenetic cycle. Papanicolau, 

 starting in each case with a winter egg, finds that as 

 successive broods are produced the color of the par- 

 thenogenetic eggs can be seen to undergo a progressive 

 change from blue to violet. As the change progresses 

 the chance that males and sexual eggs ('^ females ") will 

 appear is greater. Until towards the end of the life of 

 the individual the males and females come, as it were, 

 of themselves (Fig. 94). If, however, individuals of 

 successive broods are subjected to cold, it is found that 

 while earlier broods do not respond, later ones respond 



