CHAP. XII.] 



243 



Graciosa, St. George's, Pico, and Fayal ; and a western of two, 

 Flores and Corvo. Now had the whole group once been united 

 to the continent, or even formed parts of one extensive Atlantic 

 island, we should certainly expect the central group, which is 

 more compact and has a much larger area than all the rest, to 

 have the greatest number and variety of birds. But the fact 

 that birds are most numerous in the eastern group, and diminish 

 as we go westward, is entirely opposed to this theory, while it is 

 strictly in accordance with the view that they are all stragglers from 

 Europe, Africa, or the other Atlantic islands. Omitting oceanic 

 wanderers, and including all birds which have probably arrived in- 

 voluntarily, the numbers are found to be forty species in the eastern 

 group, thirty-six in the central, and twenty-nine in the western. 



To account for the presence of one peculiar species — the 

 bullfinch (which, however, does not differ from the common 

 European bullfinch more than do some of the varieties of 

 North American birds from tlieir type-species) is not difficult ; 

 the wonder rather being that there are not more peculiar forms. 

 In our third chapter we have seen how great is the amount of 

 individual variation in birds, and how readily local varieties 

 become established wherever the physical conditions are suffi- 

 ciently distinct. Now we can hardly have a greater difference of 

 conditions than between the continent of Europe or North Africa, 

 and a group of rocky islands in mid- Atlantic, situated in the full 

 course of the Gulf Stream and with an excessively mild though 

 stormy climate. We have every reason to believe that special 

 modifications would soon become established in any animals 

 completely isolated under such conditions. But they are not, 

 as a rule, thus completely isolated, because, as we have seen, 

 stragglers arrive at short intervals ; and these, mixing with the 

 residents, keep up the purity of the breed. It follows, that only 

 those species which reach the Azores at very remote intervals 

 will be likely to acquire well-marked distinctive characters ; 

 and this appears to have happened with the bullfinch alone, a 

 bird which does not migrate, and is therefore less likely to be 

 blown out to sea, more especially as it inhabits woody districts. 

 A few other Azorean birds, however, exhibit slight differences 

 from their European allies. 



R 2 



