330 
ISLAND LIFE. 
[part II. 
knowledge), the birds of Formosa show a far greater number 
of peculiar species than those of Japan. 
General remarks on recent Continental Islands . — We have now 
briefly sketched the zoological peculiarities of an illustrative 
series of recent continental islands, commencing with one of the 
most recent — Great Britain — in which the process of formation 
of peculiar species has only just commenced, and terminating 
with Formosa, probably one of the most ancient of the series, 
and which accordingly presents us with a very large proportion 
of peculiar species, not only in its mammalia, which have no 
means of crossing the wide strait which separates it from the 
mainland, but also in its birds, many of which are quite able to 
cross over. 
Here, too, we obtain a glimpse of the way in which species 
die out and are replaced by others, which quite agrees with 
what the theory of evolution assures us must have occurred. 
On a continent, the process of extinction will generally take 
effect on the circumference of the area of distribution, because 
it is there that the species comes into contact with such adverse 
conditions or competing forms as prevent it from advancing 
further. A very slight change will evidently turn the scale and 
cause the species to contract its range, and this usually goes on 
till it is reduced to a very restricted area, and finally becomes 
extinct. It may conceivably happen (and almost certainly has 
sometimes happened) that the process of restriction of range by 
adverse conditions may act in one direction only, and over a 
limited district, so as ultimately to divide the specific area into 
two separated parts, in each of which a portion of the species 
will continue to maintain itself. We have seen that there is 
reason to believe that this has occurred in a very few cases both 
in North America and in Northern Asia. (See pp. 64-66.) But 
the same thing has certainly occurred in a considerable number 
of cases, only it has resulted in the divided areas being occupied 
by representative forms instead of by the very same species. The 
cause of this is very easy to understand. We have already 
shown that there is a large amount of local variation in a 
