232 THE DOCTRINE OF DESCENT, 



existed since the origin of the species. Among the 

 smaller mammals, a considerable portion are common 

 to each island and the continent ; but the vast physical 

 changes that must have occurred during the breaking up 

 and subsidence of such extensive regions have led to the 

 extinction of some in one or more of the islands, and 

 in some cases there seems also to have been time for a 

 change of species to have taken place. Birds and insects 

 illustrate the same view, for every family, and almost 

 every genus of these groups found in any of the islands, 

 occurs also on the Asiatic continent, and in a great num- 

 ber of cases the species are exactly identical. Birds 

 offer us one of the best means of determining the law 

 of distribution ; for though at first sight it would appear 

 that the vv^atery boundaries which keep out the land quad- 

 rupeds could be easily passed over by birds, yet prac- 

 tically it is not so ; for if we leave out the aquatic tribes 

 which are pre-eminently wanderers, it is found that the 

 others (and especially the Passeres, or true perching 

 birds, which form the vast majority) are generally as 

 strictly limited by straits and arms of the sea as are 

 quadrupeds themselves. As an instance, among the 

 islands of which I am now speaking, it is a remarkable 

 fact that Java posesses numerous birds which never pass 

 over to Sumatra, though they are separated by a strait 

 only fifteen miles wide, and with islands in mid-channel. 

 Java, in fact, possesses more birds and insects peculiar to 

 itself than either Sumatra or Borneo, and this would 

 indicate that it was earliest separated from the con- 

 tinent; next in organic individuality is Borneo; while 

 Sumatra is so nearly identical in all its animal forms 

 with the peninsula of Malacca, that we may safely 



