s more favoured rivals, though
its extreme rarity (only one specimen having been seen to many hundreds
of the other form) would indicate that it may soon become extinct.

The only other case of polymorphism in the genus Papilio, at all equal
in interest to those I have now brought forward, occurs in America; and
we have, fortunately, accurate information about it. Papilio Turnus is
common over almost the whole of temperate North America; and the female
resembles the male very closely. A totally different-looking insect both
in form and colour, Papilio Glaucus, inhabits the same region; and
though, down to the time when Boisduval published his "Species General,"
no connexion was supposed to exist between the two species, it is now
well ascertained that P. Glaucus is a second female form of P. Turnus.
In the "Proceedings of the Entomological Society of Philadelphia," Jan.,
1863, Mr. Walsh gives a very interesting account of the distribution of
this species. He tells us that in the New England States and in New York
all the females are yellow, while in Illinois and further south all are
black; in the intermediate region both black and yellow females occur in
varying proportions. Lat. 37 deg. is approximately the southern limit of the
yellow form, and 42 deg. the northern limit of the black form; and, to
render the proof complete, both black and yellow insects have been bred
from a single batch of eggs. He further states that, out of thousands
of specimens, he has never seen or heard of intermediate varieties
between these forms. In this interesting example we see the effects of
latitude in determining the proportions in which the individuals of each
form should exist. The conditions are _here_ favourable to the one form,
_there_ to the other; but we are by no means to suppose that these
conditions consist in climate alone. It is highly probable that the
existence of enemies, and of competing forms of life, may be the main
determining influences; and it is much to be wished that such a
competent observer as Mr. Walsh would endeavour to ascertain what are
the adverse causes which are most efficient in keeping down the numbers
of each of these contrasted forms.

Dimorphism of this kind in the animal kingdom does not seem to have any
direct relations to the reproductive powers, as Mr. Darwin has shown to
be the case in plants, nor does it appear to be very general. One other
case only is known to me in another family of my eas