er variety of climate as well as a much larger area, possesses
about an equal number of mammalia with Formosa, and an even larger
proportion of peculiar species. Its birds, however, though more numerous
are less peculiar; and this is probably due to the large number of species
which migrate northwards in summer, and find it easy to enter Japan through
the Kurile Isles or Saghalien.[96] Japan too, is largely peopled by those
northern types which have an unusually wide range, and which, being almost
all migratory, are accustomed to cross over seas of moderate extent. The
regular or occasional influx of these species prevents the formation of
special insular races, such as are almost always produced when a portion of
the population of a species remains for a considerable time completely
isolated. We thus have explained the curious fact, that while the mammalia
of the two islands are almost equally peculiar, (those of Japan being most
so in the present state of our 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 {409} 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