Iv HUMMING-BIRDS 329 
colony is formed, any stray bird which may come over adds 
to the numbers, and checks permanent variation by cross- 
breeding. 
We find, then, that all the chief peculiarities of the three 
allied species of humming-birds which inhabit the Juan 
Fernandez group of islands, may be fairly traced to the action 
of those general laws which Mr. Darwin and others have 
shown to determine the variation of animals and the per- 
petuation of those variations. It is also instructive to note 
that where the variations of colour and size have been greatest 
they are accompanied by several lesser variations in other 
characters. In the Juan Fernandez bird the bill has become 
a little shorter, the tail feathers somewhat broader, and the 
fiery cap on the head somewhat smaller ; all these peculiarities 
being less developed or absent in the birds inhabiting Mas- 
afuera. These coincident changes may be due, either to 
what Mr. Darwin has termed correlation of growth, or to 
the partial reappearance of ancestral characters under more 
favourable conditions, or to the direct action of changes of 
climate and of food; but they show us how varied and un- 
accountable are the changes in specific forms that may be 
effected in a comparatively short time, and by means of very 
slight changes of locality. 
If now we consider the enormously varied conditions 
presented by the whole continent of America—the hot, moist, 
and uniform forest-plains of the Amazon; the open Ilanos of 
the Orinoco ; the dry uplands of Brazil; the sheltered valleys 
and forest slopes of the Eastern Andes; the verdant plateaux, 
the barren paramos, the countless volcanic cones with their 
peculiar Alpine vegetation; the contrasts of the east and 
west coasts ; the isolation of the West Indian islands, and to 
a less extent of Central America and Mexico, which we know 
have been several times separated from South America; and 
when we further consider that all these characteristically 
distinct areas have been subject to cosmical and local changes, 
to elevations and depressions, to diminution and increase of 
size, to greater extremes and greater uniformity of temper- 
ature, to increase or decrease of rainfall; and that with these 
changes there have been coincident changes of vegetation and 
of animal life, all affecting in countless ways the growth and 
