common in so many male animals, are due to the presence and retention
by them of the spermatic fluid; but this can hardly be the case; for
many male birds, for instance young pheasants, become brightly coloured
in the autumn of their first year.) In mankind, and even as low down in
the organic scale as in the Lepidoptera, the temperature of the body is
higher in the male than in the female, accompanied in the case of man
by a slower pulse. (30. For mankind, see Dr. J. Stockton Hough, whose
conclusions are given in the Popular Science Review, 1874, p. 97. See
Girard s observations on the Lepidoptera, as given in the Zoological
Record, 1869, p. 347.) On the whole the expenditure of matter and
force by the two sexes is probably nearly equal, though effected in
very different ways and at different rates.
From the causes just specified the two sexes can hardly fail to differ
somewhat in constitution, at least during the breeding-season; and,
although they may be subjected to exactly the same conditions, they
will tend to vary in a different manner. If such variations are of no
service to either sex, they will not be accumulated and increased by
sexual or natural selection. Nevertheless, they may become permanent if
the exciting cause acts permanently; and in accordance with a frequent
form of inheritance they may be transmitted to that sex alone in which
they first appeared. In this case the two sexes will come to present
permanent, yet unimportant, differences of character. For instance, Mr.
Allen shews that with a large number of birds inhabiting the northern
and southern United States, the specimens from the south are
darker-coloured than those from the north; and this seems to be the
direct result of the difference in temperature, light, etc., between
the two regions. Now, in some few cases, the two sexes of the same
species appear to have been differently affected; in the Agelaeus
phoeniceus the males have had their colours greatly intensified in the
south; whereas with Cardinalis virginianus it is the females which have
been thus affected; with Quiscalus major the females have been rendered
extremely variable in tint, whilst the males remain nearly uniform.
(31. Mammals and Birds of E. Florida, pp. 234, 280, 295.)
A few exceptional cases occur in various classes of animals, in which
the females instead of the males have acquired well pronounced
secondary sexual characters, such as brighter colours, greater size,
strength, or pugnacity. With birds there has sometimes been a complete
transposition of the ordinary characters proper to each sex; the
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