allied Mergus cucullatus we have, however, a case of this kind: the two
sexes differ conspicuously in general plumage, and to a considerable
degree in the speculum, which is pure white in the male and
greyish-white in the female. Now the young males at first entirely
resemble the females, and have a greyish-white speculum, which becomes
pure white at an earlier age than that at which the adult male acquires
his other and more strongly-marked sexual differences: see Audubon,
Ornithological Biography, vol. iii. 1835, pp. 249-250.) Between such
extreme cases of close sexual resemblance and wide dissimilarity, as
those of the Crossoptilon and peacock, many intermediate ones could be
given, in which the characters follow our two rules in their order of
development.
As most insects emerge from the pupal state in a mature condition, it
is doubtful whether the period of development can determine the
transference of their characters to one or to both sexes. But we do not
know that the coloured scales, for instance, in two species of
butterflies, in one of which the sexes differ in colour, whilst in the
other they are alike, are developed at the same relative age in the
cocoon. Nor do we know whether all the scales are simultaneously
developed on the wings of the same species of butterfly, in which
certain coloured marks are confined to one sex, whilst others are
common to both sexes. A difference of this kind in the period of
development is not so improbable as it may at first appear; for with
the Orthoptera, which assume their adult state, not by a single
metamorphosis, but by a succession of moults, the young males of some
species at first resemble the females, and acquire their distinctive
masculine characters only at a later moult. Strictly analogous cases
occur at the successive moults of certain male crustaceans.
We have as yet considered the transference of characters, relatively to
their period of development, only in species in a natural state; we
will now turn to domesticated animals, and first touch on monstrosities
and diseases. The presence of supernumerary digits, and the absence of
certain phalanges, must be determined at an early embryonic period the
tendency to profuse bleeding is at least congenital, as is probably
colour-blindness yet these peculiarities, and other similar ones, are
often limited in their transmission to one sex; so that the rule that
characters, developed at an early period, tend to be transmitted to
both sexes, here wholly fails. But this rule, as before remarked, does
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