CHAP. XVIII JAPAN AND FORMOSA 408 



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 accord- 

 ingly 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 

 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 some- 

 times 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 



