d 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 involuntarily, 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 sufficiently distinct. Now
we can hardly have a greater difference of conditions {252} 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.

There is another reason for the very slight amount of peculiarity presented
by the fauna of the Azores as compared with many other oceanic islands,
dependent on its comparatively recent origin. The islands themselves may be
of considerable ant