ISOLATION 177 



Island are half blue and half orange. The southern 

 tits (Myiomoira) have a yellow breast, and those in 

 the North Island a white one. The thrushes 

 (Turnagra) and robins (Miro) also differ in the two 

 islands. We cannot suppose that these different 

 species formerly inhabited different districts, when 

 the two islands were united, that is, before Cook's 

 Strait was formed ; because part of the South Island 

 lies to the north of part of the North Island, and if 

 the two islands were again united the birds of Nelson 

 and Wellington would certainly mix together. 

 Some of these species must evidently have been 

 formed since the islands were separated, and cannot 

 be due to external conditions. If we turn our at- 

 tention to the Chatham Islands we find that the 

 bell-bird (Anthornis), the fern-bird (Sphenceacus) t 

 the warbler (Pseudogerygone), and the pigeon 

 (Hemiphaga) , all differ from their relations in New 

 Zealand. It is unnecessary to discuss all these 

 cases, but I will take the Chatham Island pigeon 

 as an example. It differs from that of New Zealand 

 in having the lower part of the back and the outer 

 wing-coverts grey instead of purple. We have no 

 reason to suppose that these colours are in any way 

 related to the environment. We have no reason to 

 suppose that the Chatham Island bird could not live 

 equally well in New Zealand, or that the New 

 Zealand pigeon would be under any disadvantage in 

 the Chatham Islands. Consequently, we have no 

 reason for thinking that these differences of plumage 

 are of any special use to the birds, or that they could 

 have been produced by natural selection ; and the 



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