IV 
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,” Macmillan' s Magazine, September 1877, pp 
394-398, and chapter v., post. 
