COURTSHIP AND SEX 235 



Darwin estimated the ratio of male chicks to female 

 chicks at 48*64 : 51*36 ; and Professor Raymond Pearl 

 (1917), working on a basis of 22,000 chicks, found the ratio 

 to be 48*57 : 5i"43, a very close confirmation. Prenatal 

 mortaUty in the fowl is not differential in respect to sex, and 

 in consequence the observed sex-ratio at birth is to be 

 regarded as substantially the same as the ratio established in 

 the fertilised eggs. This is not always the case in animals, 

 for male embryos may prove more delicate than females. 

 Professor Pearl points out the great importance of knowing 

 the precise sex-ratio before making deductions in regard 

 to sex- determination. Before changes in the sex-ratio can 

 be regarded as indicative of either environmental effects or 

 hereditary bias to male-production or female-production, 

 it is necessary to show that the changes occur with such 

 frequency as to exceed considerably that expected on the 

 basis of chance alone. 



As regards birds, we may almost dismiss the theory that 

 nurture is a directly determining factor. The sex is settled 

 very early in embryonic life, and there is no evidence of an 

 indifferent period. That peculiarities of nurture may operate 

 indirectly will be seen later on. 



(II.) Possible Importance of the Relative Condition o£ 

 the Germ-Cells at Fertilisation, — Many breeders believe 

 that there is significance in the relative ages of the parents 

 at the time of fertilisation, in the relative vigour of the 

 parents, in the relative ages of the germ-cells. Schultze's 

 prolonged experiments with mice gave no support to any 

 of these opinions ; but Hertwig and Kuschakewitsch found 

 that either over-ripeness or under-ripeness of the eggs (due 

 to artificially delaying or hastening fertilisation) led to a 

 large excess of females. Kuschakewitsch, working with ova 

 with a high degree of over- ripeness, actually obtained cultures 

 of male tadpoles only, and with only 4*6 per cent, of deaths. 

 These factors are not known to apply in the case of Birds. 



(III.) Are there Pre-determined Male-Producing and 

 Female-Producing Germ Cells? — In the vine-pest Phyl- 

 loxera among insects and in Hydatina among Rotifers, there 



