Chap. VIII. 
SEXUAL SELECTION. 
303 
sufficient evidence, with respect to man and certain 
domesticated animals, to shew that this is one impor- 
tant 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 polygamy has been supposed to 
lead to the birth of a greater proportion of female 
infants; but Dr. J. Campbell 39 carefully 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 offspring are almost exactly equal in 
number. 
Eorses. — Mr. Tegetmeier lias been so kind as to tabulate for me 
from the 1 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 that year published. The total births have 
been 25, 560, 40 consisting of 12,763 males and 12,797 females, or in 
the proportion of 99*7 males to 100 females. As these numbers are 
tolerably large, and as they are drawn from all parts of England, 
during several years, we may with much confidence conclude that 
with the domestic horse, or at least with the race-horse, the two 
sexes are produced in almost equal numbers. The fluctuations in 
the proportions during successive years are closely like those which 
•occur with mankind, when a small and thinly-populated area is con- 
sidered : thus in 1856 the male horses were as 107*1, and in 1867 
as only 92*6 to 100 females. In the tabulated returns the propor- 
tions vary in cycles, for the males exceeded the females during six 
successive years ; and the females exceeded the males during two 
39 Anthropological Review, April, 1870, p. cviii. 
40 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 shewing how infertile these highly-nurtured and rather 
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 produce offspring. 
During 1867, 836 males and 902 females were born, and 794 mares failed. 
