PDF - Wallace Online

HUMMING-BIRDStwo red males differ very slightly from each other, but thethree green females differ considerably and the curiousjpointis that the female in the smaller and more distant island somewhatresembles the same sex in Chili, while the female of theJuan Fernandez species is very distinct, although the malesof the two islands are so much alike. As this forms a comparativelysimple case of the action of the laws of variationand natural selection, it will be instructive to see if we canpicture to ourselves the process by which the changes havebeen brought about. We must first go back to an unknownbut rather remote period, just before any humming-birds hadreached these islands. At that time a species of this peculiargenus, Eustephanus, must have inhabited Chili but we cannotbe sure that it was identically the same as that which is;now found there, because we know that species are alwaysundergoing change to a greater or less degree. After perhapsmany failures, one or more pairs of the Chilian bird got blownacross to Juan Fernandez, and finding the country favourable,with plenty of forests and a fair abundance of flowers andinsects, they rapidly increased and permanently establishedthemselves on the island. They soon began to change colour,however, "the male getting a tinge of reddish-brown, whichgradually deepened into the fine colour now exhibited by thetwo insular species, while the female, more slowly, changedto white on the under - surface and on the tail, while thebreast -spots became more brilliant. When the change ofcolour was completed in the male, but only partially so in thefemale, a further emigration westward took place to thesmall island Mas-afuera, where they also established themselves.Here, however, the change begun in the larger islandappears to have been checked, for the female remains to thisday intermediate between the Juan Fernandez and the Chilianforms. More recently, the parent form has again migratedfrom Chili to Juan Fernandez, where it still lives side by sidewith its greatly changed descendant. 1 Let us now see howfar these facts are in accordance with the general laws of1 In the preceding account of the probable course of events in peoplingthese islands with humming-birds, I follow Mr. Sclater's paper on the"Land Birds of Juan Fernandez," Ibis, 1871, p. 183. In what follows Igive my own explanation of the probable causes of the change.