butterflies,
which really belong to very distinct and not at all closely allied
genera. Other important characters are--(1) The existence of a small
basal cell in the hind wings of Ituna which is wanting in Thyridia; (2)
the division of the cell between the veins 1_b_ and 2 of the hind wings
in the former genus, while it is undivided in the latter; and (3) the
existence in Thyridia of scent-producing tufts of hair on the upper edge
of the hind wing, while in Ituna these are wanting; but in place of them
are extensible processes at the end of the abdomen, also emitting a
powerful scent. These differences characterise two marked subdivisions
of the Danaoid Heliconinae, each containing several distinct genera; and
these subdivisions are further distinguished by very different forms of
larvae, that to which Ituna belongs having from two to four long
threadlike tentacles on the back, while in that containing Thyridia
these are always absent. The former usually feed on Asclepiadeae, the
latter on Solanaceae or Scrophulariaceae.

The two species figured, though belonging to such distinct and even
remote genera, have acquired almost identical tints and markings so as
to be deceptively alike. The surface of the wings is, in both,
transparent yellowish, with black transverse bands and white marginal
spots, while both have similar black-and white-marked bodies and long
yellow antennae. Dr. Mueller states that they both show a preference for
the same flowers growing on the edges of the forest paths.[105]

We will now proceed to give the explanation of these curious
similarities, which have remained a complete puzzle for twenty years.
Mr. Bates, when first describing them, suggested that they might be due
to some form of parallel variation dependent on climatic influences; and
I myself adduced other cases of coincident local modifications of
colour, which did not appear to be explicable by any form of
mimicry.[106] But we neither of us hit upon the simple explanation given
by Dr. Fritz Mueller in 1879.

His theory is founded on the assumed, but probable, fact, that
insect-eating birds only learn by experience to distinguish the edible
from the inedible butterflies, and in doing so necessarily sacrifice a
certain number of the latter. The quantity of insectivorous birds in
tropical America is enormous; and the number of young birds which every
year have to learn wisdom by experience, as regards the species of
butterflies to be ca