CHAP. XVIII.] 
JAPAN AND FORMOSA. 
381 
considerable number of species, and we may be sure that were 
it not for the constant intermingling and intercrossing of the 
individuals inhabiting adjacent localities this tendency to local 
variation would soon form distinct races. But as soon as the 
area is divided into two portions the intercrossing is stopped, 
and the usual result is that two closely allied races, classed 
as representative species, become formed. Such pairs of allied 
species on the two sides of a continent, or in two detached areas, 
are very numerous ; and their existence is only explicable on the 
supposition that they are descendants of a parent form which 
once occupied an area comprising that of both of them, — that 
this area then became discontinuous, — and, lastly, that, as a 
consequence of the discontinuity, the two sections of the parent 
species became segregated into distinct races or new species. 
Now, when the division of the area leaves one portion of the 
species in an island, a similar modification of the species, either 
in the island or in the continent, occurs, resulting in closely- 
allied but distinct forms ; and such forms are, as we have seen, 
highly characteristic of island-faunas. But islands also favour 
the occasional preservation of the unchanged species — a pheno- 
menon which very rarely occurs in continents. This is probably 
due to the absence of competition in islands, so that the parent 
species there maintains itself unchanged, while the continental 
portion, by the force of that competition, is driven back to some 
remote mountain area, where it too obtains a comparative free- 
dom from competition. Thus may be explained the curious fact, 
that the species common to Formosa and India are generally 
confined to limited areas in the Himalayas, or in other cases are 
found only in remote islands, as Japan or Hainan. 
The distribution and affinities of the animals of continental 
islands thus throws much light on that obscure subject — the 
decay and extinction of species ; while the numerous and delicate 
gradations in the modification of the continental species, from 
perfect identity, through slight varieties, local forms, and insular 
races, to well-defined species and even distinct genera, afford 
an overwhelming mass of evidence in favour of the theory of 
“ descent with modification.” 
