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HUMMING-BIRDS 327tains itself ;for Mr. Moseley, of the Challenger expedition, hasinformed the writer that humming-birds are extraordinarilyabundant in Juan Fernandez, every bush or tree having oneor two darting about it.Here, then, we have one of thespecial conditions which have always been held to favourvariation a great increase in the number of individuals ; but,as there was no struggle with allied creatures, there was noneed for any modification in form or structure, and we accordinglyfind that the only important variations which havebecome permanent are those of size and of colour. Theincreased size would naturally arise from greater abundanceof food with a more equable climate throughout the year ;the healthier, stronger, and larger individuals being preserved.The change of colour would depend on molecular changes inthe plumage accompanying the increase of size and the;superior energy and vitality in the male, aided by the favourablechange in conditions and rapid increase of population,would lead to an increased intensity of colour, the special tintbeing determined either by local conditions or by inheritedtendencies in the race. It is to be noted that the changefrom green to red is in the direction of the less refrangiblerays of the spectrum, and is in accordance with the law ofchange which has been shown to accompany expansion ininorganic growth and development in organic forms. 1 Thechange of colour in the female, not being urged on by suchintense vital activity as in the case of the male, would bemuch slower, and, owing probably to inherited tendencies, ina different direction. The under-surface of the Chilian birdisashy with bronzy-green spots on the breast, while the tailis entirely bronze-green.In the Juan Fernandez species theunder-surface has become pure white, the breast-spots largerand of a purer golden-green,while the whole inner web of thetail-feathers has become pure white, producing a most eleganteffect when the tail is expanded.We may now follow the two sexes to the remoter island,at a period when the male had acquired his permanent styleof colouring, but was not quite so large as he subsequentlybecame ;while the change of the female bird had not been1See "Colours of Animals," MacmiUan's Magazine, September 1877, pp.394-398, and chapter v., post.