TWO PAIRS OF FEMALE TWINS 233 



It is clear, therefore, that these data justify the preceding state- 

 ment on the effects of hybridity upon the normal size relations 

 of the two yolks (ova) of the pigeon's clutch. They also empha- 

 size the fact that the two twin-bearing eggs were from bird 

 groups which assuredly throw a very high proportion of larger 

 yolks in the second egg of the clutch. Both twin-producing 

 yolks were, as already noted, the second of the clutch. 



The two series of breeding records which supplied these two 

 cases of twins (tables 1 and 2) afford an opportunity partially to 

 illustrate still another condition which affects the sex production, 

 and to a certain extent the storage capacities, of the ova of pig- 

 eons. In earlier papers we have referred to 'crowded reproduc- 

 tion' merely as an aspect of 'reproductive overwork.' We 

 can here particularize to the extent indicated in tables 1 to 4. 



The summaries at the bottom of tables 1 and 2, and the four 

 divisions (columns 2 and 3) of tables 3 and 4, show that the sex 

 ratio changes with the rate at which the eggs are produced. In 

 the case of 9 A248 (table 1) those clutches which were separated 

 from the preceding clutch by an interval of eight days or more 

 yielded 26 d^ : 13 9 (sex unknown 9); those of seven-day inter- 

 vals, 9 cf : 23 9 (unknown 8) ; and those of six-day intervals, 

 4 cf : 8 9 (unknown 0). For the hybrid 9 60 (table 2), the corre- 

 sponding figures are : 26 cf : 27 9 (unknown 8) ; 5 cf : 11 9 (un- 

 known 2); 2 0?" :99 (unknown 5). Under the most crowded 

 reproduction (six and seven days) in these two series it is clear 

 that there is an undoubted deficiency of males, even if all of the 

 eggs of unknown sex value were classed as males. The fact 

 that one pair of t^\ ins arose from a clutch with a six-day interval 

 (table 1) and the other from a seven-day interval (table 2) is 

 therefore significant for the purposes of the present paper. The 

 actual time intervals involved predisposed, so to speak, these 

 clutches to femininity, and both our pubHshed and unpublished 

 data show conclusively that femininity is correlated with a high 

 storage metabolism of the ova. 



Tables 3 and 4 supply still a different method of analysis of the 

 ••elation of 'crowded reproduction' to sex. In column 2 of those 

 tables the whole period of egg laying (shown in tables 1 and 2) 



