330 



ISLAND LIFE. 



[part II. 



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 accordingly 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 fiaally 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 tKe 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 



