The Descent of Man, and Selection in Relation to SexDarwin, Charles
History
The Descent of Man, and Selection in Relation to Sex
Darwin, Charles
Evolution (Biology); Human beings -- Origin; Sex differences; Sexual dimorphism (Animals); Sexual selection in animals
The sexes of sheep are not ascertained by agriculturists until several
months after birth, at the period when the males are castrated; so that
the following returns do not give the proportions at birth. Moreover, I
find that several great breeders in Scotland, who annually raise some
thousand sheep, are firmly convinced that a larger proportion of males
than of females die during the first year or two. Therefore the
proportion of males would be somewhat larger at birth than at the age
of castration. This is a remarkable coincidence with what, as we have
seen, occurs with mankind, and both cases probably depend on the same
cause. I have received returns from four gentlemen in England who have
bred Lowland sheep, chiefly Leicesters, during the last ten to sixteen
years; they amount altogether to 8965 births, consisting of 4407 males
and 4558 females; that is in the proportion of 96.7 males to 100
females. With respect to Cheviot and black-faced sheep bred in
Scotland, I have received returns from six breeders, two of them on a
large scale, chiefly for the years 1867-1869, but some of the returns
extend back to 1862. The total number recorded amounts to 50,685,
consisting of 25,071 males and 25,614 females or in the proportion of
97.9 males to 100 females. If we take the English and Scotch returns
together, the total number amounts to 59,650, consisting of 29,478
males and 30,172 females, or as 97.7 to 100. So that with sheep at the
age of castration the females are certainly in excess of the males, but
probably this would not hold good at birth. (59. I am much indebted to
Mr. Cupples for having procured for me the above returns from Scotland,
as well as some of the following returns on cattle. Mr. R. Elliot, of
Laighwood, first called my attention to the premature deaths of the
males, —a statement subsequently confirmed by Mr. Aitchison and others.
To this latter gentleman, and to Mr. Payan, I owe my thanks for large
returns as to sheep.)
Of CATTLE I have received returns from nine gentlemen of 982 births,
too few to be trusted; these consisted of 477 bull-calves and 505
cow-calves; i.e., in the proportion of 94.4 males to 100 females. The
Rev. W.D. Fox informs me that in 1867 out of 34 calves born on a farm
in Derbyshire only one was a bull. Mr. Harrison Weir has enquired from
several breeders of PIGS, and most of them estimate the male to the
female births as about 7 to 6. This same gentleman has bred RABBITS for
many years, and has noticed that a far greater number of bucks are
produced than does. But estimations are of little value.
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