290 THE EVOLUTION THEORY 



selection. Any one who has seen Gould's magnificent collection of 

 humming-birds in the British Museum in London knows what 

 a surprising diversity of red, green, and blue metallic brilliance these 

 birds display, what contrasts are to be found in the diverse colour- 

 schemes, and what differences they exhibit in the length and form of 

 the feathers of the head, of the neck, of the breast, and especially of 

 the tail. There are wedge-shaped, evenly truncate, and deeply forked 

 tails, some with single long, barbless feathers, and so on. All these 

 characters are confined to the males, and are at most only hinted at 

 in the female ; in no species does the female even remotely approach 

 the male in brilliance or decora tiveness of plumage. 



I do not believe that so many species with very divergent plumage in 

 the males could have developed if they had all lived together on a large 

 connected area. But here, distributed over a large number of isolated 

 mountain forests, the decorative colouring or the distinctive shape which 

 chances to arise through germinal selection on any of these terrestrial 

 islands can go on increasing, undisturbed by crossing with indi\'iduals 

 of the ancestral species, and furthered, moreover, by sexual selection. 



In this way, if I mistake not, numerous new species have arisen 

 as a result of isolation, and it is quite intelligible that several new 

 species may have arisen from one and the same ancestral species, as 

 we may see from the nearly related yet constantly different species 

 of mocking-bird on the different islands of the Galapagos group. 



A number of similar examples might be given from among 

 birds. Thus Dixon calls attention to the species of the thrush genus 

 Catharus, twelve of which live in the mountain forests of Mexico 

 and of South America as far as Bolivia, all differing only slightly from 

 one another and all locally separated. They came from the plains, 

 migrated to the highlands, were isolated there, and then no longer 

 varied together all in the same direction, but each isolated group 

 evolved in a different direction according to the occurrence of chance 

 germinal variations : one developed a chestnut-brown head, another 

 a slate-grey mantle, a third a brown-red mantle, and so on. From 

 what we have already seen in regard to the importance of sexual 

 selection in evolving the [plumage of birds, it is probable that this 

 factor has been operative in this case also. 



Another example is afforded by the weaver-birds (Ploceus) of 

 South Africa, those ingenious singing-birds resembling blackbirds 

 in size and form, Avhose pouch-shaped nests, hanging freely from 

 a branch, usually over the water, and with their little openings on 

 the under side, are excellently protected from almost every form 

 of persecution. These birds have in South Africa split up into twenty 



