124 CARNEGIE INSTITUTION OF WASHINGTON. 



fewer survived this treatment; also, when subjected to cooHng, more 

 males were killed and fewer (in proportion to total) survived. In all 

 these respects Dr. Riddle is convinced that the metabolic theory of 

 sex is supported. The pigeons, therefore, have now supplied cogent 

 evidence of fundamental, metabolic sexual difference in their germinal, 

 embryonic, and adult stages. 



MODIFICATION OF THE SEX-RATIO IN MAN. 



The standard sex-ratio may be considered to be 100 males to 100 

 females. In some species where the ratio deviates far from 100, a 

 special explanation is demanded and has sometimes been received. 

 The sex-ratio in man is usually over lOO^not far from 105 — and this 

 deviation is doubtless due either to the fact that male-producing sperm 

 have a better chance of fertilizing the egg than female-producing sperm, 

 or else that the male embryo (zygote) is more viable, on the average, 

 than the female embryo (zygote). Dr. Little is paying special atten- 

 tion to the human sex-ratio, and his work was reported on in the Year 

 Book for 1919 (pp. 135-137). It was there pointed out that the sex- 

 ratio is greater when the parents belong to different European races 

 than when they belong to the same, as 122 is to 106. It now appears 

 that when both parents are whites born in the United States the ratio 

 is high (118), which we might expect in view of the hybrid nature of 

 our white population. Studies on the sex-ratio in the colored popula- 

 tion yield some new and unexpected facts. The sex-ratio of the off- 

 spring of colored parents born in the United States is exceptionally 

 low (96). Of offspring of colored parents, born in the British West 

 Indies, and probably less hybrid than the progeny of colored persons 

 born in the United States, the sex-ratio is 108. If we may regard the 

 West Indians as less hybrid, then on the basis of the findings in Euro- 

 peans we should expect a smaller ratio than in offspring of colored 

 Americans. While in white primapara the sex-ratio is higher in 

 first births than in offspring of subsequent births as 115.5 =±=1.5 is to 

 97.3 ±1.2, in the colored population the sex-ratio is the lower in first 

 births as 103.6 ±2.8 is to 11 2.0 ±2.8. 



SEX-LINKED LETHAL FACTORS IN MICE. 



One important cause of disturbance of the sex-ratio is the presence 

 of some lethal factor in, or the absence of some vital factor from, the 

 sex-chromosome. Dr. Little has found the sex-ratio of inbred, non- 

 waltzing mice to be 103.1 ±2.8, giving the usual slight excess of males. 

 The sex-ratio of litters from a closely inbred race of Japanese waltzing 

 mice is 53.2 ±5.7. The difference between the two sex-ratios is 7.9 

 times its probable error, so it is certainly significant. Reciprocal 

 crosses of animals from this particular strain of inbred waltzing mice 

 with non-waltzing races give extremely interesting and distinct results. 



