176 MR. A. B. 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 and Hornbills, and two Celebes birds of the 

 Asiatic genera JPhcenicophcBus 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 to have become extinct, and to have been replaced by an allied 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 has 

 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. Gynopithecus, a genus of Baboons, the 

 extraordinary Babirusa and the singular ruminant Ansa de'pres- 

 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 



