238 THE BIOLOGY OF BIRDS 



this is of interest because the more hydrated state of the 

 colloids favours a higher rate of oxidising metabolism, while 

 the less hydrated state favours increased storage. An 

 estimate of the energy-value of the yolks, determined by 

 the calorimeter, confirms these conclusions as to the funda- 

 mental contrast between male-producing and female- 

 producing eggs. 



Dr. Riddle's thesis is corroborated by curious facts in 

 regard to sex-behaviour. Thus, females hatched from eggs 

 laid earlier in the season are more masculine in their sex- 

 behaviour than are their own full sisters hatched later in the 

 season. Several grades of females can be arranged in a 

 series according to the season of hatching. The female 

 hatched from the first egg of a clutch is in the majority of 

 cases more masculine than her sister hatched from the second 

 of the clutch. And another curious point is the frequency 

 with which the right ovary persists in birds hatched from 

 eggs otherwise known to be most biased towards femaleness. 



It seems that sex and such qualities as fertility and 

 developmental energy not only bear initial relations to the 

 order of the egg in the clutch, but that they can be pro- 

 gressively modified under stress of reproductive overwork^ 

 until at the extreme end of the season certain feminine 

 features are abnormally or unusually accentuated. Thus a 

 hereditary character, such as fertility, may have some degree 

 of plasticity — of quantitative plasticity at least. 



The experiments outlined above point to a very important 

 conclusion : that sex is a quantitative modifiable character, 

 associated with modifiable metabolic levels. Femaleness in 

 the egg is associated with low metabolism, lower percentage 

 of water, and higher total of fat and phosphorus, or of phos- 

 phatides ; and conversely for maleness. Analyses of the 

 blood of adult birds go to show that the metabolic diflPerences 

 of male and female germs are also expressed in the constitu- 

 tion of the adult birds. In the adult female the blood- 

 plasma is richer in alcohol-soluble substance and phosphorus 

 than in the adult male, and this increases during sexual 

 activity. 



