PERIODICITY IN THE PRODUCTION OF MALES. 1 93 



No statement here made is to be construed, however, as a 

 contradiction of my former claim that external conditions may 

 alter the extent of male production.^ Few biological facts are 

 more firmly established than that external factors modify the 

 life cycle of Hydatina. The results described above merely 

 show that, under fairly uniform conditions, there is nevertheless 

 a periodicity in the production of males which must be due to 

 internal factors. 



Periodicity and Nutrition. 



Mitchell ('13) has pointed out that in Asplanchna periods of 

 male production are also often, perhaps usually, periods of 

 vigorous growth and rapid reproduction ; and he concludes there- 

 from that male production is a result of high nutrition. This 

 conclusion may be correct, but it is scarcely logical, since co- 

 incident events are not always related to one another as cause 

 and effect. But assuming as Mitchell does that size of family 

 is a guide to nutrition, let us examine all the sources of informa- 

 tion that are extensive enough to be of value, to determine, if 

 possible in this way, the relation of nutrition to male production. 

 In my own work in the past few years, there have been two lines 

 in which hundreds of families have been reared. By collecting 

 all of the families of the same size in a single line, and recording 

 the proportion of male-producers, it should be possible to discover 

 to what extent size of family and male production are correlated. 

 Obviously one must not collect in the same group families belong- 

 ing to two or more unrelated lines, for one of these lines may 

 have larger families, and at the same time (but from other causes) 

 either many or few male-producers, so that the groups of families 

 of large size would have on the average a correspondingly high 

 or low proportion of male-producers. Such an apparent correla- 

 tion would have no significance. Within a single line, however, 

 no such error could affect the results. The two tables herewith 

 presented (IV. and V.) are each compiled from families belonging 

 to a single line. 



1 My discovery several years ago of internal differences between parthenogenetic 

 lines of Hydatina senta, the result of which is a different proportion of male pro- 

 ducers in each line, is characterized by Mitchell as a "return to the position of 

 Punnett." Since Punnett never found an effect of external conditions, and since 

 I never repudiated my experiments proving the effect of external conditions, there 

 can have been no "return." 



