From Alfred Russel Wallace [December? 1860]1
From “Von Buch on the Flora of the Canaries”.2 “On continents the individuals of one kind of plant disperse themselves very far, and by the difference of stations of nourishment & of soil produce varieties which at such a distance not being crossed by other varieties and so brought back to the primitive type, become at length permanent and distinct species. Then if by chance in other directions they meet with an other variety equally changed in its march, the two are become very distinct species and are no longer susceptible of intermixture.”
P.S. “Natural Selection” explains almost everything in Nature, but there is one class of phenomena I cannot bring under it,—the repetition of the forms & colours of animals in distinct groups, but the two always occurring in the same country & generally on the very same spot.3 These are most striking in insects, & I am constantly meeting with fresh instances. Moths resemble butterflies of the same country—Papilios in the east resemble Euplœ as, in America Heliconias. At Amboyna I took on the same tree at the same time two longicorns of distinct genera, but so alike in colour & markings that I only separated them after some days— Here also & at Macassar occurs together a Malacoderm & an Elater of exactly the same tints of metallic blue & soft orange & also similarly striate,— yet there is no affinity between them. A few days ago only I took a new & curious little Cicindela which so closely resembles in size & markings a Therates occurring with it, that I never know which it is till I take it out of my net;—yet there is no sign of a change in the structural characters which separate these genera. It seems to shew that colour, markings & texture of surface depend strictly on local conditions— Home Entomologists might do something in experiments on breeding insects, varying conditions of food light heat &c as much as they will bear. In domestic vars. have you discovered what tends to produce white, black, or particular coloured variations? or what tends to produce spots rather than stripes.4
Gives an extract from L. von Buch on the flora of the Canaries [Physikalische Beschreibung der Canarische Inseln (1825)].
Natural selection does not explain why animals of different groups in the same place often resemble each other.