409 



ISLAND LIFE 



PART II 



Asia. {See pp. 65 — 68.) But the same thing has certainly 

 occurred in a considerable number of cases, only it has 

 resulted in the divided areas being occupied by representa- 

 tive 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 

 considerable number of species, and we may be sure that 

 were it not for the constant intermingling and inter- 

 crossing of the individuals inhabiting adjacent localities, 

 such variations, being selected in adaptation to divergent 

 conditions, 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 con- 

 tinent, 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 con- 

 sequence of the discontinuity, the two sections of the 

 parent species became segregated through adaptation into 

 distinct races or new species. 



Now, w^hen 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 phenomenon 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 con- 

 tinental portion, by the force of that competition, is driven 

 back to some remote mountain area, where it also obtains 

 a comparative freedom 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. 



