Iv HUMMING-BIRDS 325 
two red males differ very slightly from each other, but the 
three green females differ considerably ; and the curious point 
is that the female in the smaller and more distant island some- 
what resembles the same sex in Chili, while the female of the 
Juan Fernandez species is very distinct, although the males 
of the two islands are so much alike. As this forms a com- 
paratively simple case of the action of the laws of variation 
and natural selection, it will be instructive to see if we can 
picture to ourselves the process by which the changes have 
been brought about. We must first go back to an unknown 
but rather remote period, just before any humming-birds had 
reached these islands. At that time a species of this peculiar 
genus, Eustephanus, must have inhabited Chili; but we can- 
not be sure that it was identically the same as that which is 
now found there, because we know that species are always 
undergoing change to a greater or less degree. After perhaps 
many failures, one or more pairs of the Chilian bird got blown 
across to Juan Fernandez, and finding the country favourable, 
with plenty of forests and a fair abundance of flowers and 
insects, they rapidly increased and permanently established 
themselves on the island. They soon began to change colour, 
however, the male getting a tinge of reddish-brown, which 
gradually deepened into the fine colour now exhibited by the 
two insular species, while the female, more slowly, changed 
to white on the under-surface and on the tail, while the 
breast- spots became more brilliant. When the change of 
colour was completed in the male, but only partially so in the 
female, a further emigration westward took place to the 
small island Mas-afuera, where they also established them- 
selves. Here, however, the change begun in the larger island 
appears to have been checked, for the female remains to this 
day intermediate between the Juan Fernandez and the Chilian 
forms. More recently, the parent form has again migrated 
from Chili to Juan Fernandez, where it still lives side by side 
with its greatly changed descendant.! Let us now see how 
far these facts are in accordance with the general laws of 
1 In the preceding account of the probable course of events in peopling 
these islands with humming- birds, I follow Mr. Sclater’s paper on the 
‘‘ Land Birds of Juan Fernandez,” Jbis, 1871, p. 183. In what follows T 
give my own explanation of the probable causes of the change. 
