328 TROPICAL NATURE IV 
half completed. In this small and comparatively barren 
island (a mere rock, as it is described by some authors) there 
would be no such constant abundance of food, and therefore 
no possibility of a large permanent population of humming- 
birds; while the climate would not differ materially from 
that of the larger island. Variation would therefore be 
checked, or might be stopped altogether; and we find the 
facts exactly correspond to this view. The male, which had 
already acquired his colour, remains almost undistinguishable 
from his immediate ancestral form; but he is a little smaller, 
indicating either that the full size of that form had not been 
acquired at the period of migration, or that a slight diminution 
of size has since occurred, owing to a deficiency of food. The 
female shows also a slight diminution of size, but in other 
respects is almost exactly intermediate between the Chilian and 
Juan Fernandez females. The colour beneath is light ashy, the 
breast-spots are intermediate in size and colour, and the tail- 
feathers have a large ill-defined white spot on the end of the 
inner web which has only to be extended along the whole web 
to produce the exact character which has been acquired in 
Juan Fernandez. It seems probable, therefore, that the 
female bird has remained nearly or quite stationary since its 
migration, while its Juan Fernandez relative has gone on 
steadily changing in the direction already begun; and the 
more distant species geographically thus appears to be more 
nearly related to its Chilian ancestor. 
Coming down to a more recent period, we find that the 
comparatively small and dull-coloured Chilian bird has again 
migrated to Juan Fernandez; but it at once came into 
competition with its red descendant, which had firm possession 
of the soil, and had probably undergone slight constitutional 
changes exactly fitting it to its insular abode. The new- 
comer, accordingly, only just manages to maintain its footing ; 
for we are told by Mr. Reed of Santiago that it is by no 
means common ; whereas, as we have seen, the red species is 
excessively abundant. We may further suspect that the 
Chilian birds now pass over pretty frequently to Juan 
Fernandez, and thus keep up the stock; for it must be 
remembered that whereas, at a first migration, both a male 
and a female are necessary for colonisation, yet, after a 
