riddle: control of sex ratio 327 



of males is produced. It may be noted in passing that this 

 generalization touches the question of the nature of sexual differ- 

 ence; for, studies among the most diverse animals and plants 

 have afforded evidences of the "increased vigor of hybrids," of 

 what Darwin called the "good effects of crossing," and of what 

 has been observed in Mendelian breeding as the "greater vigor 

 of the heterozygote." The means of "increasing the vigor" of 

 the offspring are, therefore, the very same means by which 

 higher and higher proportions of males are obtained; and males, 

 we have concluded from other studies, are characterized by a 

 more active metabolism than that found in females'. 



A glance at Table 3 will assist in making clear some of the 

 advantages which the pigeons afford in the analysis of sex ratios. 

 First, examining the details of the "family cross" — it is an excep- 

 tionally bad history with almost complete infertility — we note 

 that only males are produced, but that a very great number of 

 eggs failed completely to develop. It might be contended that 

 in such a series only the male-producing eggs are fertilized, and 

 for this reason only males are produced. We may fully grant 

 the point; though attention should be directed to the fact that 

 if this were the whole of the story it is rather remarkable that 

 only 4 eggs of the 18 here shown (6 of 88 in the entire series) were 

 fertilized, since it can be proved in any similar series that at least 

 half of the 18 eggs (also half of the 88) were male-producing 

 eggs. And a further point of interest is that while 4 of the first 

 18 eggs were fertile only 2 of the last 70 eggs — produced under 

 overwork, or crowded reproduction — were fertile. But to recur 

 to the original point — the pigeon in any event affords an oppor- 

 tunity to study the total production of the animal's ovary; and 

 this particular animal's ovary contains all of the sexually differ- 

 entiated germs. 



In the second section of Table 3 are given the details of 

 a generic cross, a cross of less widely departed forms than in the 

 preceding case. In these crosses practically every egg can be 

 hatched and the sex of the resulting offspring learned. This was 

 done in 23 of the 24 eggs here recorded. This particular record 

 is one of the many made by Professor Whitman from which he 



