HUMMING-BIRDS 327 



tains itself ; for Mr. Moseley, of the Challenger expedition, has 

 informed the writer that humming-birds are extraordinarily 

 abundant in Juan Fernandez, every bush or tree having one 

 or two darting about it. Here, then, we have one of the 

 special conditions which have always been held to favour 

 variation a great increase in the number of individuals ; but, 

 as there was no struggle with allied creatures, there was no 

 need for any modification in form or structure, and we accord- 

 ingly find that the only important variations which have 

 become permanent are those of size and of colour. The 

 increased size would naturally arise from greater abundance 

 of 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 in 

 the plumage accompanying the increase of size ; and the 

 superior energy and vitality in the male, aided by the favour- 

 able change in conditions and rapid increase of population, 

 would lead to an increased intensity of colour, the special tint 

 being determined either by local conditions or by inherited 

 tendencies in the race. It is to be noted that the change 

 from green to red is in the direction of the less refrangible 

 rays of the spectrum, and is in accordance with the law of 

 change which has been shown to accompany expansion in 

 inorganic growth and development in organic forms. 1 The 

 change of colour in the female, not being urged on by such 

 intense vital activity as in the case of the male, would be 

 much slower, and, owing probably to inherited tendencies, in 

 a different direction. The under-surface of the Chilian bird 

 is ashy with bronzy-green spots on the breast, while the tail 

 is entirely bronze-green. In the Juan Fernandez species the 

 under-surface has become pure white, the breast-spots larger 

 and of a purer golden-green, while the whole inner web of the 

 tail-feathers has become pure white, producing a most elegant 

 effect 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 style 

 of colouring, but was not quite so large as he subsequently 

 became ; while the change of the female bird had not been 



1 See "Colours of Animals," MacmiUan's Magazine, September 1877, pp. 

 394-398, and chapter v., post. 



