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 watery 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 saiely 
