the male which has been chiefly modified through sexual selection; the
amount of difference between the sexes mostly depending on the form of
inheritance which has prevailed. Inheritance is governed by so many
unknown laws or conditions, that it seems to us to act in a capricious
manner (28. The Variation of Animals and Plants under Domestication,
vol. ii. chap. xii. p. 17.); and we can thus, to a certain extent,
understand how it is that with closely allied species the sexes either
differ to an astonishing degree, or are identical in colour. As all the
successive steps in the process of variation are necessarily
transmitted through the female, a greater or less number of such steps
might readily become developed in her; and thus we can understand the
frequent gradations from an extreme difference to none at all between
the sexes of allied species. These cases of gradation, it may be added,
are much too common to favour the supposition that we here see females
actually undergoing the process of transition and losing their
brightness for the sake of protection; for we have every reason to
conclude that at any one time the greater number of species are in a
fixed condition.
MIMICRY.
This principle was first made clear in an admirable paper by Mr. Bates
(29. Transact. Linn. Soc. vol. xxiii. 1862, p. 495.), who thus threw
a flood of light on many obscure problems. It had previously been
observed that certain butterflies in S. America belonging to quite
distinct families, resembled the Heliconidae so closely in every stripe
and shade of colour, that they could not be distinguished save by an
experienced entomologist. As the Heliconidae are coloured in their
usual manner, whilst the others depart from the usual colouring of the
groups to which they belong, it is clear that the latter are the
imitators, and the Heliconidae the imitated. Mr. Bates further observed
that the imitating species are comparatively rare, whilst the imitated
abound, and that the two sets live mingled together. From the fact of
the Heliconidae being conspicuous and beautiful insects, yet so
numerous in individuals and species, he concluded that they must be
protected from the attacks of enemies by some secretion or odour; and
this conclusion has now been amply confirmed (30. Proc. Entomological
Soc. Dec. 3, 1866, p. xlv.), especially by Mr. Belt. Hence Mr. Bates
inferred that the butterflies which imitate the protected species have
acquired their present marvellously deceptive appearance through
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