March 12, 1915] 



SCIENCE 



403 



females will be found in their descendants 

 than in any other possible case. From such 

 eom,binations were derived nine animals, all 

 males. In the same way, from the combina- 

 tion a male derived from a father, male tend- 

 ency, and mother, female tendency, mated 

 with a female from father, female tendency 

 and mother male tendency, were derived six 

 animals all of which were females. 



This regulation in the inheritance of the 

 sex tendency is especially interesting in af- 

 fording an explanation .of the manner in 

 which the equilibrium is maintained between 

 the number of male and female offspring of a 

 given species. With each new generation 

 each male animal has an opposite tendency 

 from that of his father and each female ani- 

 mal an opposite tendency from that of her 

 mother. It, therefore, follows that a disturb- 

 ance of the equilibrium in one generation will 

 tend to be restored by the opposite tendencies 

 in the following generation. The above-men- 

 tioned change of sex tendency from litter to 

 litter in the female leads to the same result. 

 This third factor " C "' regulates equilibrium 

 from birth to birth so that any disturbance of 

 a great degree is impossible. 



The difference in the proportion between the 

 sexes in different species may be due to the 

 fact that in some species the father and 

 mother have an equal influence on the deter- 

 mination of the sex of their offspring while 

 in other species either the father or the 

 mother may have the greater influence. 



When father and mother have equal influ- 

 ence, the eonibination father, male tendency, 

 with mother, female tendency, will give equal 

 number males with females tendencies and fe- 

 males with male tendency, and the combina- 

 tion father, female tendency, with mother, 

 male tendency, will give equal number males 

 with male tendency and females with fe- 

 male tendency. In this way equal numbers 

 of male and female descendants will be pro- 

 duced and equal numbers of the descendants 

 will have male and female tendencies. In 

 such a case the sex ratio should be 100 per 

 cent. Should, on the other hand, the mother 

 have the greater influence on the determina- 



tion of sex, as seems to be the case in the 

 guinea-pigs, then the number of descendants 

 with a male tendency will be greater than the 

 number of those having a female tendency, as 

 the following scheme shows: 



Father with male tendency mated with 

 mother with female tendency will give more 

 females (male tendency) than males (female 

 tendency). 



Father with a female tendency mated with 

 a mother with a male tendency gives more 

 males (male tendency) than females (female 

 tendency). Therefore from either combina- 

 tion the greater number of offspring have a 

 male tendency and as a result of this the sex 

 ratio will be greater than 100. 



In guinea-pigs it really seems that the in- 

 fluence of the mother is greater than that of 

 the father, and this may be the explanation of 

 the fact that the number of male guinea-pigs 

 is greater than the number of females. 



Finally, if the father has a greater influence 

 on sex determination than the mother the 

 number of descendants with a female tendency 

 will be greater than the number with a male 

 tendency, and consequently the sex ratio will 

 be smaller than 100, as is shown by the fol- 

 lowing analysis: 



Father with male tendency mated with 

 mother with female tendency gives more males 

 (female tendency) than females (male tend- 

 ency). The father with female tendency 

 mated with mother with male tendency gives 

 more females (female tendency) than males 

 (male tendency). Thus from either combina- 

 tion the greater number of offspring have a 

 female-producing tendency and as a result of 

 this the sex ratio will be less than 100. 



Concerning the third factor, " C," the 

 change of the sex tendency from litter to 

 litter, a statistical examination shows the fol- 

 lowing results : 



First : Relative number of male and female 

 descendants after the birth of one or more 

 females in one litter; 38 males to 12 females, 

 i. e., 76 per cent, males to 24 per cent, females 

 (sex ratio 316.66). 



Second: Relative number of male and fe- 

 male descendants after birth of one or more 



