TWO PAIRS OF FEMALE TWINS 231 



ent sperm from the same hybrid male maj^ exercise opposite 

 tendencies for the production of sex.^ A probable instance (Q 

 of 265) of this is partiall}^ described in the footnote to table 8. 



The females which produced the twins, and the eggs listed 

 in tables 1 and 2, were mated to blond ring-dove (Streptopelia 

 risoria) males. Indeed, these males are sire and son; and, further 

 in one of the series the sire is mated to his daughter (A248). 

 The hybrid female (alba-risoria No. 60) is thus mated with one 

 of her parent species, and the two species which enter into her 

 composition are closely related ones. In consequence both of 

 these female tend — aside from special modifying conditions — 

 to throw higher proportions of males from first eggs of the clutch 

 and of females from the second eggs of the clutch (tables 3 and 4) . 

 The bisexual clutches (those producing the two sexes) are con- 

 sidered in tables 5 and 6. Among these latter this proportion is 

 5 : 1, or 5 : 3 (?) for female A248,6 and 12 : 7 for female 60. 



Reference to table 7 will show that all pairs of eggs obtained 

 from pure blond rings which differed by as much as 20 per cent 

 in weight had yolk weights which differed in the same sense as 

 the egg weights; i.e., in this pure species, the yolk of the second 

 egg was invariably (seven cases) larger than the yolk of its clutch 

 mate, when the total weight of the second egg was 20 or more 

 per cent larger than the egg weight of the clutch mate. In the 

 case of the twin-producing egg belonging to this series the egg- 

 weight difference was 43.1 per cent; this very wide difference fully 

 guarantees the larger size of the yolk which it contained and which 

 gave rise to the twins. 



The difference (43.1 per cent) is also far greater than that of any 

 of the thirty-five pairs of yolks weight of alba-risoria hybrids 

 given in table 8. That table shows that every egg-weight differ- 

 ence of more than 14 per cent correctly indicated the direction 

 of the difference between the pairs of yolks produced by these 



^ The factual support of this unorthodoxy must, after consulting the papers 

 cited under note 2, in part await the publication of, a) C. O. Whitman, Posthu- 

 mous Works, vol. II (The Carnegie Institution, in press) and, b) our own forth- 

 coming work. 



^ Some evidence from inbred relatives of this bird possibly indicate a slight 

 contamination of St. alba in this female. 



