TWO PAIRS OF FEMALE TWINS 229 



plete in table 1. The similar record of the hybrid* female (No. 

 60) is given in table 2. A glance at those two tables will show 

 how conspicuously larger is each of the twin-producing eggs than 

 is any other egg of the series to which it belongs; the tables give 

 the necessary details for one hundred^ eggs of one series and 

 ninety-seven in the other. 



The extraordinarily large size of the eggs which produced the 

 twins, in comparison with all of the other eggs produced by these 

 particular parents (totals of 116 and 134 eggs), is itself sufficient 

 to make it extremely probable that the yolks contained within 

 them were of very large size. The certainty of their large yolk 

 size becomes apparent in the light of the results of our accurate 

 measurements on many thousands of eggs. It is, of course, im- 

 possible to give all of these latter measurements here; this is also 

 unnecessary since these will appear in connection with the com- 

 plete account of our studies on sex in pigeons. It does seem 

 necessary, however, to give here the particular segment of this 

 evidence which is contained in the several following tables. 



Reference to tables 5 and 6 will show that the eggs which pro- 

 duced the twins were, in one case, 24.9 per cent, and in the other, 

 43.1 per cent larger than the associated egg (of the same clutch). 

 Such amounts of difference between the two eggs of the clutch are 

 highly abnormal; except in cases of evident dwarfing of the smaller 

 egg of the pair they practically do not exist. In the two pairs of 

 twin-producing eggs it will be observed that the smaller eggs of 

 the pairs are not dwarfed. A number of cases approximating 

 to these extreme differences have, however, appeared in our 

 records. An assistant has gone through the entire body of our 

 breeding records and listed all pairs of eggs in which the members 

 of the pair differ by 20 per cent or more (tables 7 to 12). In 

 connection with these summaries it was thought advantageous 



* This female is hybrid between two very closely related ring-doves — Strep- 

 topelia alba and St. risoria (| alba, ^ risoria). 



* Sixteen additional eggs (to December 1) have since been laid by female 

 A248; the largest egg weight among these is 9.40 grams. Thirty-seven eggs 

 have since been added to the series containing ninety-seven eggs, and the largest 

 egg weight among these is 9.11 grams. Most of the tables of this paper are sum- 

 maries of data collected to April, 1917. 



