LIFE CYCLE OF HYDATINA SENTA 157 



completely established. In like manner, it can no longer be 

 doubted that, as intimated above, internal factors may exist 

 which modify that ratio. This is of particular importance because 

 of the tendency which has existed to attribute all the phenomena 

 either to external factors alone or to internal factors alone. Little 

 can yet be said regarding the nature of the internal differences. 

 Punnett suggested that the ratio of male-producers in a pure line 

 depends upon the character of the zygote from which the pure 

 line springs, and the nature of the zygote in turn depends of 

 course upon the nature of the gametes. Were it not for the possi- 

 bihty of modifying that ratio by external conditions, much in the 

 experiments described in this paper might seem to support Pun- 

 nett's view. When two unrelated individuals, coming from two 

 pure lines which behaved differently with respect to the propor- 

 tion of male-producers, were crossed, the zygote gave rise to a 

 pure line which, when reared under the same conditions as the 

 parent lines, yielded a higher proportion of male-producers than 

 either parent line. The same result was obtained in every experi- 

 ment of this kind. But when two rather closely related individ- 

 uals, one from the Fi line just mentioned, the other from one of 

 the original parent lines (Experiment XXXVI), were mated, the 

 zygote gave rise to a pure line having a proportion of male-pro- 

 ducers intermediate between those of its two parent lines. If it 

 had been found impossible to alter these results by employing 

 different external agents in the different pure lines, we might per- 

 haps be justified in saying that zygotic constitution is alone re- 

 sponsible for the ratio of male-producers. It was possible, how- 

 ever, to reverse the results. The Fi pure line which was yielding 

 more male-producers than either of its parent lines, was made, 

 when reared in beef extract, to yield fewer male-producers than 

 its parent lines. Other things being equal, some internal factor, 

 perhaps zygotic constitution, causes different pure lines to yield 

 different proportions of male-producers ; but Punnett's assumption 

 that zygotic constitution determines that proportion regardless of 

 external conditions is not justified. There are no pure ''strains" 

 producing 40 per cent of male-producers or 2 per cent. A line 

 producing 40 per cent can be made to yield 20 per cent as long as 



