176 
MR. A. R. WALLACE ON THE ZOOLOGICAL 
one to the other. In like manner the cases of identical species 
in the eastern and western islands of the Archipelago are due to 
the gradual and accidental commingling of originally absolutely 
distinct faunas. 
In our second class (representative species) we must place the Wild 
Pigs, which seem to be of distinct but closely allied species in each 
island ; the Squirrels also of Celebes are of peculiar species, as are 
the Woodpeckers aud HornbOls, and two Celebes birds of the 
Asiatic genera PTuenicopheeus and Acridotheres. Now these and 
a few more of like character are closely allied to other species in- 
habiting Java, Borneo, or the Philippines. We have only there- 
fore to suppose that the species of the western passed over to the 
eastern islands at so remote a period as on one side or the other 
to have become extinct, and to have been replaced by an allied 
form, and we shall have produced exactly the state of things now 
existing. Such extinction and such replacement we know lias 
been continually going on. Such has been the regular course of 
nature for countless ages in every part of the earth of which we 
have geological records ; and unless we are prepared to show that 
the Indo- Australian Archipelago was an altogether exceptional 
region, such must have been the course of nature here also. If 
these islands have existed in their present form only during one 
of the later divisions of the Tertiary period, and if interchange of 
species at very rare and distant intervals has occurred, then the 
fact of some identical and other closely allied species is a necessary 
result, even if the two regions in question had been originally 
peopled by absolutely distinct creations of organic beings, and 
there had never been any closer connexion between them than 
now exists. The occurrence of a limited number of representative 
species in the two divisions of the Archipelago does not therefore 
prove any true transition from one to the other. 
The examples of our third class — of peculiar genera having little 
or no affinity with those of the adjacent islands — are almost entirely 
confined to Celebes, and render that island a district per se, in the 
highest degree interesting. Chjnopithecus , a genus of Baboons, the 
extraordinary Babirusa and the singular ruminant Ansa depres- 
sicornis have nothing in common with Asiatic mammals, but seem 
more allied to those of Africa. A quadrumanous animal of the 
same genus (perhaps identical) occurs in the little island of Bat- 
chian, which forms the extreme eastern limit of the highest order 
of mammalia. An allied species is also said to exist in the Philip- 
pines. Now this occurrence of quadrumana in the Australian 
