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Show 330 ISLAND LIFE. [PAHT lf, knowledge), the birds of Formosa show a far greater number of peculiar species than those of Japan. General TemaTlcs on 1·ecent Continental Islands.-We have now briefly sketched the zoological peculiarities. of a? illustrative series of recent continental islands, commencmg with one of the most recent-Great Britain-in which the process of formation of peculiar species has only just commenced~ and terminat~ng with Formosa, probably one of the most anCient of the senes, and which accordingly presents us with a very large proportion of peculiar species, not only in. its n~ammalia, whi~h 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 qccurred. On a continent, the process of extinction will generally take effect on the circumference of the area of distributiou, because it is there that the species comes into contact with such ad verse 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 thjs 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 ~IIAP. XVI!!.) .JAPAN AND FORMOSA. 381 considerable number of species, n.nd. we may be sure that were it not for the constant intermingling and intcrcrossing of the individuals inhabiting adjacent localities this tendency to local variation woul<l Roon form distinct races. But as soon as the area is dividecl 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 t\VO 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 compritsing that of both of them,-t.hat this area then became discontinuous,-and, lastly, that, as a consequence of the discontinuity, the two sections of the parent species became segregated into 0istinct races or new species. Now, when the division of the area leaves one portion of the Rpecies in an island, a similar modification of the species, either in the island or in the continent, occurs, resulting in closelyallied 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 continental portion, by the force of that competition, is driven back to some remote mountain ·area, where it too 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. 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." |