o 



26 Experifnental Zoology 



The results are summed up as follows: Toward the end of the 

 season an individual may first give birth for a few days to a mix- 

 ture of parthenogenetic and sexual individuals, but at this time 

 the males predominate as a rule. The parthenogenetic young 

 now cease to appear, and only sexual individuals are born. 

 After two or three days of mixed sexual broods, the males cease 

 to appear, and there then follows a long series of sexual 

 females, that terminates only with the death of the mother. 

 Not more than 20 per cent of the sexual forms were males 

 in one of the species studied by Balbiani, and many of these 

 died in the larval stages. Certain females begin at once to 

 produce sexual forms without having first produced partheno- 

 genetic young. 



Balbiani isolated young females about to give birth to sexual 

 young, and placed them on young branches of the same plant 

 in order to see whether the kind of young produced is affected 

 by the condition of the food plant. They continued, despite 

 the change, to produce sexual individuals throughout the rest 

 of their fives. Conversely, a female producing parthenogenetic 

 young was placed on a branch cut from the plant. She con- 

 tinued to produce parthenogenetic young. As the branch be- 

 came dry she acquired wings and flew away. Balbiani concludes 

 that food does not produce any effect on the mode of reproduc- 

 tion unless the organism is ''predisposed to submit to its influ- 

 ence." This predisposition appears at certain times of year 

 characteristic for each species. Possibly the results may be 

 interpreted, I think, to mean that external conditions require 

 more than one generation to produce their eft'ects, and w^hen once 

 effected the mode of reproduction for that individual cannot be 

 altered. It remains to be shown, I think, that there is an in- 

 born predisposition that is independent of external conditions to 

 produce sexual forms. On the contrary, the possibifity of pro- 

 longing the parthenogenetic series indefinitely by proper condi- 

 tions shows that the predisposition is connected with external 

 changes. 



Stevens has recently confirmed Balbiani's discovery that the 



