HUMMING-BIRDS 



two red males differ very slightly from each other, but the 

 three green females differ considerably j 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. 1 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," Ibis, 1871, p. 183. In what follows I 

 give my own explanation of the probable causes of the change. 



