Chap. VIII.] SEXUAL SELECTION. 293 



It has often been supposed that the relative ages of 

 the parents determine the sex of the offspring ; and Prof. 

 Leuckart ^® has advanced what he considers sufficient evi- 

 dence, with respect to man and certain domesticated ani- 

 mals, to show that this is one important factor in the 

 result. So, again, the period of impregnation has been 

 thought to be the efficient cause ; but recent observations 

 discountenance this belief. Again, with mankind polyg- 

 amy has been supposed to lead to the birth of a greater 

 proportion of female infants ; but Dr. J. Campbell^" care- 

 fully attended to this subject in the harems of Siam, and 

 he concludes that the proportion of male to female births 

 is the same as from monogamous unions. Hardly any 

 animal has been rendered so highly polygamous as our 

 English race-horses, and we shall immediately see that 

 their male and female offsjn-ing are almost exactly equal 

 in number. 



Horses. — Mr. Tegetraeier has been so kind as to tabulate for 

 me from the ' Racing Calendar ' the births of race-horses during 

 a period of twenty-one years, viz., from 1846 to 1867; 1849 being 

 omitted, as no returns were tliat year published. The total births 

 have been 25,560,*° consisting of 12,763 males and 12,797 females, 

 or in the proportion of 99.7 males to 100 females. As these num- 



Azara ('Voyages dans I'Amerique merid.,' tom. ii. 1809, pp. 60, 170), 

 the women in proportion to the men are as 14 to 13. 



2S Leuckart (in Wagner, ' Handveorterbuch der Phys.' B. iv. 1853, s. 

 774). 



2^ Anthropological Eeview, April, 1870, p. cviii. 



*^ During the last eleven years a record has been kept of the number 

 of mares which have proved barren or prematurely slipped their foals ; 

 and it deserves notice, as showing how infertile these highly-nurtured 

 and I'ather closely-interbred animals have become, that not far from one- 

 third of the mares failed to produce living foals. Thus, during 1866, 809 

 male colts and 816 female colts were born, and 743 mares failed to pro- 

 duce offspring. During 1867, 836 males and 902 females were born, and 

 79-Jl mares failed. 



