Page:The naturalist on the River Amazons 1863 v1.djvu/289

 acting directly on individuals had originally produced this race or species, they certainly would have caused much modification of it in different parts of this region; for the upper Amazons country differs greatly from the district near the Atlantic in climate, sequence of seasons, soil, forest clothing, periodical inundations, and so forth. These differences moreover graduate away, so that the species is subjected to a great diversity of physical conditions from locality to locality, and ought in consequence to present an endless series of local varieties, on the view mentioned, instead of one constant form throughout. Besides, how should we explain the fact of H. Thelxiope and H. Melpomene both existing under the same local conditions; and how account for the diversified modifications presented in one and the same locality as at Serpa and on the Tapajos?

There is evidently therefore some more subtle agency at work in the segregation of a race than the direct operation of external conditions. The principle of natural selection, as lately propounded by Darwin, seems to offer an intelligible explanation of the facts. According to this theory, the variable state of the species exhibited in the districts above mentioned would be owing to Heliconius Melpomene having been rendered vaguely instable by the indirect action of local conditions dis-