Page:Darwinism by Alfred Wallace 1889.djvu/274

 difference, in the important character of the neuration of the wings, between these 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 1b 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. Müller states that they both show a preference for the same flowers growing on the edges of the forest paths.

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. But we neither of us hit upon the simple explanation given by Dr. Fritz Müller in 1879.

His theory is founded on the assumed, but probable,