922 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Boys born to 1000 Girls born (1887-91). 



Italy 1058 



Ireland 1055 



German Empire 1052 



France . .1046 



England 1036 



Connecticut 1072 



Ehode Island 1049 



Massachusetts 1046 



The proportional birth-rate of the two sexes is usually fairly constant from 

 year to year. This means that constant regulating factors are at work. 

 What determines sex in any one individual is ill understood. The sexual 

 organs in the human embryo are well differentiated at the eighth week of 

 intra-uterine life, hence the sex of the child must be settled previously to this 

 time. It is at present quite impossible to say whether it is settled in the 

 germ-cells previous to their union, in the act of fertilization, or during the 

 early uterine life. Many facts, both observational and experimental, and 

 more hypotheses, bearing upon the determination of sex, have been brought 

 forward. The Hofacker-Sadler law (Hofacker, 1828; Sadler, 1830) is well 

 known, as follows : If the father be older than the mother, more boys than 

 girls will be born ; if the parents be of equal age, slightly more girls than 

 boys ; if the mother be older than the father, the probability of girls is still 

 greater. Since its promulgation this so-called law has received evidence both 

 confirmatory and contradictory of its truth. Thury in 1863 claimed that the 

 earlier after its liberation the egg is fertilized, the greater is the tendency to 

 the production of a female ; the later the fertilization, the greater the prob- 

 ability of a male. Breeders have made use of this principle apparently with 

 success offspring conceived at the beginning of "heat" seem to be more 

 usually females. Likewise, it is frequently believed that in human beings 

 conceptions immediately after menstruation produce a larger proportion of 

 females than later conceptions. Diising * accepts Thury 's view and extends it 

 to the male element the younger the spermatozoon the greater the tendency 

 toward the production of males. Hence among animals the scarcity of one 

 sex leads to the more frequent exercise of its reproductive function, the em- 

 ployment of younger germ-cells, and therefore the relative increase of that 

 sex. Further, the nearer a parent is to the height of his reproductive capacity 

 the less will be the probability of transmitting his own sex to the offspring. 

 By feeding tadpoles with highly nutritious flesh Yi.jng 2 increased the percent- 

 age of females from 56 to 92. Mrs. Treat 3 showed that the butterflies of 

 well-fed caterpillars become females, those of starved caterpillars males. Sta- 

 tistics among mammals and human beings indicate that the proportion of male 

 to female offspring varies inversely with the nutrition of the parents, especially 

 of the mother. Thus, more boys are born in the country than in the city, 

 and in poor than in prosperous families ; the relative number of boys is said to 

 vary even with the prices of food. It is contended, moreover, and with some 

 statistical support, that in the human race an epidemic or a war, either of which 

 affects adversely the well-being of the people, is followed by a relative increase 



1 K. Diising: Jenaische Zeitschrift filr Naturwissenschaft, xvi., 1883, and xvii., 1884. 



2 E. Yung : Comptes rendus de F Academic des sciences, Paris, xcii., 1881. 



3 Mrs. Mary Treat: The American Naturalist, vii., 1873. 



