PDF - Wallace Online

TROPICAL NATUREhalf completed.In this small and comparatively barrenisland (a mere rock, as it is described by some authors) therewould be no such constant abundance of food, and thereforeno of a possibility large permanent population of hummingbirds;while the climate would not differ materially fromthat of the larger island. Variation would therefore bechecked, or might be stopped altogether; and we find thefacts exactly correspond to this view. The male, which hadalready acquired his colour, remains almost undistinguishablefrom his immediate ancestral form ;but he is a little smaller,indicating either that the full size of that form had not beenacquired at the period of migration, or that a slight diminutionof size has since occurred, owing to a deficiency of food. Thefemale shows also a slight diminution of size, but in otherrespects is almost exactly intermediate between the Chilian andJuan Fernandez females. The colour beneath is light ashy, thebreast-spots are intermediate in size and colour, and the tailfeathershave a large ill-defined white spot on the end of theinner web which has only to be extended along the whole webto produce the exact character which has been acquired inJuan Fernandez. It seems probable, therefore, that thefemale bird has remained nearly or quite stationary since itsmigration, while its Juan Fernandez relative has gone onsteadily changing in the direction already begun; and themore distant species geographically thus appears to be morenearly related to its Chilian ancestor.Coming down to a more recent period, we find that thecomparatively small and dull-coloured Chilian bird has againmigrated to Juan Fernandez ;but it at once came intocompetition with its red descendant, which had firm possessionof the soil, and had probably undergone slight constitutionalchanges exactly fittingit to its insular abode. The newcomer,accordingly, only just manages to maintain its footing ;for we are told by Mr. Keed of Santiago that it is by nomeans common ; whereas, as we have seen, the red species isexcessively abundant. We may further suspect that theChilian birds now pass over pretty frequently to JuanFernandez, and thus keep up the stock; for it must beremembered that whereas, at a first migration, both a maleand a female are necessary for colonisation, yet,after a