ny birds can be explained by supposing (what there are many facts to
prove) that the females prefer the most beautiful and perfect-plumaged
males, and that thus, slight accidental variations of form and colour
have been accumulated, till they have produced the wonderful train of
the Peacock and the gorgeous plumage of the Bird of Paradise. Both these
causes have no doubt acted partially in insects, so many species
possessing horns and powerful jaws in the male sex only, and still more
frequently the males alone rejoicing in rich colours or sparkling
lustre. But there is here another cause which has led to sexual
differences, viz., a special adaptation of the sexes to diverse habits
or modes of life. This is well seen in female Butterflies (which are
generally weaker and of slower flight), often having colours better
adapted to concealment; and in certain South American species (Papilio
torquatus) the females, which inhabit the forests, resemble the AEneas
group of Papilios which abound in similar localities, while the males,
which frequent the sunny open river-banks, have a totally different
colouration. In these cases, therefore, natural selection seems to have
acted independently of sexual selection; and all such cases may be
considered as examples of the simplest dimorphism, since the offspring
never offer intermediate varieties between the parent forms.

The phenomena of dimorphism and polymorphism may be well illustrated by
supposing that a blue-eyed, flaxen-haired Saxon man had two wives, one a
black-haired, red-skinned Indian squaw, the other a woolly-headed,
sooty-skinned negress--and that instead of the children being mulattoes
of brown or dusky tints, mingling the separate characteristics of their
parents in varying degrees, all the boys should be pure Saxon boys like
their father, while the girls should altogether resemble their mothers.
This would be thought a sufficiently wonderful fact; yet the phenomena
here brought forward as existing in the insect-world are still more
extraordinary; for each mother is capable not only of producing male
offspring like the father, and female like herself, but also of
producing other females exactly like her fellow-wife, and altogether
differing from herself. If an island could be stocked with a colony of
human beings having similar physiological idiosyncrasies with Papilio
Pammon or Papilio Ormenus, we should see white men living with yellow,
red, and black women, and thei