CHAP. XVllI.] 



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." 



