CHAP. XII.] 
THE AZORES. 
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 their 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. 
