Life Cycle of Hydatma Senta 349 



of male-producers should here also appear among the 25th to 30th 

 members. But it does not; the maximum is among the 15th to 

 20th members. The same fact emerges from a comparison of 

 any other two groups in Table XV. A short family is not a cur- 

 tailed long family; it is built on a plan of its own, which is approxi- 

 mately the same, relatiit'e to its length, as that of a large family. 

 Either the family is completely worked over,or elimination occurs 

 all along the line, from beginning to end of the family, and not at 

 the end alone. 



Since in the light of new data these two proposed explanations 

 are inadequate, I have been led to seek for others. Firstly, Mau- 

 pas may have used different food for the two parts of his experi- 

 ments, but his account is too brief to enable us to judge on this 

 point. Another possible, and I believe more plausible, explana- 

 tion IS found in the effect of breeding from different parts of the 

 family. It appears that breeding from the first member of suc- 

 cessive families may yield many more male-producers than does 

 breeding from the last member. When the conditions are such 

 as to produce many male-producers among the first-born, the 

 difference may be very great, — 57 per cent and 14 per cent 

 respectively in one case. Whether this phenomenon is due to aging 

 of the food culture in the dish with the parent, or to accumulated 

 metabolic products of the rotifers themselves, or to any other 

 factor, does not concern us here. The fact remains that the first- 

 born may yield more male-producers than the last-born. If 

 Maupas reared the first five members of the family at a tempera- 

 ture of 26° to 28° C, and only decided to institute a control experi- 

 ment when it became apparent that the first families would be 

 largely male-producers, then the sister individuals used for the 

 control and placed at a temperature of 14° to 15° C, must have 

 been late members of the family. Maupas does not tell us that 

 these experiments were performed simultaneously^ and it would 

 have been very natural to have tested the high temperature to 

 see whether it offered any probable results before beginning any 

 formal experiments. I offer this explanation only as a sugges- 

 tion, but it seems to me a probable one. 



