314 T. H. MOEGAN 



males and other females produce males the fact would be in favor 

 of two lines, although this is not necessarily the conclusion to be 

 drawn from the evidence. On the other hand, if from a single fe- 

 male both sexual females and males arise the single line theory 

 will cover the case. In fact, there are a few definite records where 

 a single parthenogenetic female has been observed to give rise to 

 sexual females, males and parthenogenetic females. Balbiani 

 recorded in 1884 in the phylloxeran of the oak and of the grape 

 the birth of parthenogenetic females and males, of parthenoge- 

 netic females, of males and of sexual females, and of males and 

 sexual females from single females. 



Hunter ('00) gives a number of cases where an individual pro- 

 duces parthenogenetic young (agamic) and sexual females, other 

 records where an individual produces parthenogenetic young 

 and males, and in one record all three forms w^ere given. He 

 found in those generations where sexual forms are produced that 

 66 per cent of the offspring are agamic, that about 17 per cent 

 of the individuals are intermediate in character, i.e., they show 

 some of the characteristics of the parthenogenetic female and 

 some of the sexual females; that only 2 per cent of the individ- 

 uals born are males and 2 per cent are sexual females. The in- 

 termediate forms belong, he thinks, to the sexual generation, 

 i.e., they are not transitional forms in the sense that they 

 belong to a generation preceding the sexual generation. Their 

 occurrence is a point of great interest, for it seems to show that 

 the difference that separates the parthenogenetic from the sex- 

 ual generation is not absolutely marked off, and this would be 

 expected if environmental rather than internal factors bring 

 about the change. 



Webster and Phillips in their government report entitled "The 

 spring grain aphis or green bug, Toxoptera graminum," give 

 many records of the output of single individuals. They show 

 that one agamic female may give birth to males and partheno- 

 genetic females or to males and sexual females. Among 'aber- 

 rant individuals' they record two cases where they found in a 

 single female true eggs (i.e., sexual eggs) and living embryos 

 (i.e., parthenogenetic embryos). Such an individual is reproduc- 



