36o A HISTORY OF BIRDS 



fauna of islands derived from the neighbouring mainland. 

 These parallel factors of evolution have been by no means so 

 generally recognised as they deserve to be. In the mountains 

 of Equatorial and Eastern Africa he points out, at an altitude 

 of 3,000 feet, on Mount Elgon, Mount Kenia, Mount Kilman- 

 jaro and Mount Zomba in Nyassaland, and the mountains of 

 Shoa in Abyssinia, that many peculiar birds exist and are found 

 nowhere else in the world. For example, a species of Shrike 

 {Lanius mackinnoni) is met with on Mount Elgon, and turns 

 up again in the mountains of the Cameroons and only at high 

 altitudes. Here we have an instance where isolation has not, 

 as yet, apparently produced any superficial changes in the 

 coloration of the bird. But the beginnings of such changes 

 may be studied in Pinarochroa, a genus of Chats, which is 

 represented by a series of forms, all differing slightly one from 

 another, though so slightly as to be scarcely perceptible save 

 to the expert. These forms have been severally collected from 

 Mount Elgon, Kilmanjaro, Runenzori,and the mountains of Shoa 

 at an elevation of from 10,000 to 1 1,000 feet. In the interven- 

 ing lowlands, and even on the lower slopes of these mountains, 

 these birds are never met with ; they are as completely shut off 

 as if by an ocean. One might cite dozens of similar instances. 

 But all go to show that isolation is a factor in the production 

 of new species. 



One other instance of this kind must suffice. This is 

 furnished by certain small birds of prey known as "Falconets" 

 of the Genus Polioheirax. One species {P. semitorquatus) in- 

 habits Africa, a second species {P. insignis) is found in the 

 Burmese countries. These birds are selected because they are, 

 so to speak, " ear-marked " — if we may be permitted a Hiber- 

 nicism — by having, the male a grey and the female a chestnut 

 back, showing that they must be descendants of a common 

 ancestor which have been modified, so far as superficial char- 

 acters are concerned, by isolation. One might indeed cite 

 dozens of similar instances; but all show that isolation is at 

 least an indirect factor in the production of new species. 

 Now the foregoing facts are all the more remarkable because 

 we find that a species by no means always undergoes a change 

 when cut off from the parent stock. Isolation, in short, is, as 

 we have hinted, only a factor, and not a cause in itself, of specific 



