240 THE BIOLOGY OF BIRDS 



It is only fair to say that the importance of Riddle's 

 experiments makes it peculiarly necessary to have severe 

 testing. The experiments made by Leon J. Cole and 

 W. F. Kirkpatrick (1915) are not in agreement with those of 

 Riddle. They worked with Tumbler pigeons, and came to 

 the conclusion that the sex is determined according to the 

 laws of chance. They did not find that the first eggs tended 

 to produce males and the second eggs females. 



(IV.) Possible Indirect Influence 0! Nurture. — If there be 

 two kinds of ova produced in the ovary, and if difi^erences 

 among the spermatozoa do not affect sex in birds, it is 

 possible that peculiarities of nurture may have an indirect 

 effect by influencing the number of male-producing or 

 female-producing ova in the ovary. There may be a 

 differential elimination going on in the ovary, or it may be 

 that young oocytes (immature egg-cells in the ovary) may 

 be biased by nurtural influences towards becoming male- 

 producing or towards becoming female-producing ova. 



The normal ratios of 48*57 per cent, cockerels to 51*43 per 

 cent, pullets seems to be determined by the constitution of 

 the fertilised egg, and not by any environmental factors 

 operating during incubation. But it is possible that it may 

 be modifiable by inducing the production of ova of an 

 anabolic type and also by a change in the laying ability of 

 the hen. For, according to Raymond Pearl (1917), the 

 constitutionally more fecund hens tend to produce a larger 

 proportion of female offspring, and it is possible to breed 

 strains of poultry with high productivity as a fixed character- 

 istic. When the poultry man breeds along the right lines 

 for increased egg production, he will at the same time be 

 producing a strain in which profit-making pullets will pre- 

 ponderate over the less profitable cockerels. It may be 

 suggested that the high productivity is the outcome of a 

 constitution in which the ratio of anabolism to katabolism is 

 relatively high, and that female-producing eggs are the out- 

 come of similar physiological conditions. 



Reference may here be made to Heape's breeding 

 experiments (1907) on canaries. In one aviary the con- 



