THE AMERICAN NATURALIST [Vol.L 



order of the eggs of the clutch was found, and where the 

 worker has thought it worth while to mention the hind 

 of birds studied— such material has been wholly unsuit- 

 able to leading to a decision. That is to say, the 

 "pigeons" used in these cases were one or another of the 

 150 mongrels collectively known as domestic pigeons. 

 One of the clearest points of our present knowledge of 

 the relation of sex to egg of clutch is that the normal re- 

 lations are lost immediatrhf upon luihrUlization—i. e., in 

 passing from the pure state of the species. The count- 

 less degradations and crossings suffered by the various 

 domesticated breeds since their existence as a pure 

 species, is therefore a sufficient index of the suitability 

 of this material for a study of this subject. Whitman 

 demonstrated the predominance of males from the first, 

 and of females from the second egg of the clutch when 

 pure species mated with pure species produce the eggs, 

 and also the random distribution of the sexes from the 

 eggs of hybrids. And as early as 1911 and 1912 I dem- 

 onstrated charts and lantern slides which showed that the 

 size of the yolks from pure species showed with consid- 

 erable uniformity a smaller first, and a larger second 

 yolk; and further, that this regularity breaks down at 

 once and completely in hybrids. 3 



Let us now note the conclusions which follow upon the 

 demonstrated dimorphism of the ova 4 in the pigeons, 

 when this is reviewed in the light of breeding data on 

 these forms and in connection with the demonstrated re- 

 lationships of size of yolk to sex— relationships which 

 are continued even under the pressure brought by over- 

 work, season, and age. 



It becomes clear, first of all, that a selective fertiliza- 

 tion by one kind of sperm is quite impossible— the sex 



from the second egg of the clutch is indicated in both series. In series I, 



the male in 6 (or 7?) ca^es; to a female in 3 ises. In seffes^I the first 

 egg yielded males in 9 (or ?10) cases; females in only three cases. 



*Yolk size has now been accurately determined in about 10.000 cases. 



