326 Experimental 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 lives. 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 effects, and when 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 possibility 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 
