probably is the most efficient of all the causes of the proportion of
males to females born alive being less amongst illegitimate children
than amongst the legitimate. With most animals the greater size of the
adult male than of the female, is due to the stronger males having
conquered the weaker in their struggles for the possession of the
females, and no doubt it is owing to this fact that the two sexes of at
least some animals differ in size at birth. Thus we have the curious
fact that we may attribute the more frequent deaths of male than female
infants, especially amongst the illegitimate, at least in part to
sexual selection.
It has often been supposed that the relative age of the two parents
determine the sex of the offspring; and Prof. Leuckart (55. Leuckart,
in Wagner Handworterbuch der Phys. B. iv. 1853, s. 774.) has advanced
what he considers sufficient evidence, with respect to man and certain
domesticated animals, that this is one important though not the sole
factor in the result. So again the period of impregnation relatively to
the state of the female has been thought by some to be the efficient
cause; but recent observations discountenance this belief. According to
Dr. Stockton Hough (56. Social Science Association of Philadelphia,
1874.), the season of the year, the poverty or wealth of the parents,
residence in the country or in cities, the crossing of foreign
immigrants, etc., all influence the proportion of the sexes. With
mankind, polygamy has also been supposed to lead to the birth of a
greater proportion of female infants; but Dr. J. Campbell (57.
Anthropological Review, April 1870, p. cviii.) carefully attended to
this subject in the harems of Siam, and 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 the English
race-horse, and we shall immediately see that his male and female
offspring are almost exactly equal in number. I will now give the facts
which I have collected with respect to the proportional numbers of the
sexes of various animals; and will then briefly discuss how far
selection has come into play in determining the result.
HORSES.
Mr. Tegetmeier 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
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