180 MR. A. E. WALLACE ON THE ZOOLOaiCAL 



tributiou of the genus Fai'ttdisea, two species of wliich (the com- 

 mon Birds of Paradise) are found only in New Guinea and the 

 islands of Aru, Mysol, Waigiou, and Jobie, all of which are con- 

 nected with New Guinea by banks of soundings, while they do 

 not extend to Ceram or the Ke Islands, which are no further from 

 New Guinea, but are separated from it by deep sea. Again, the 

 chain of small volcanic islands to the west of Gilolo, though divided 

 by channels of only ten or fifteen miles wide, possess many distinct 

 representative species of insects, and even, in some cases, of birds 

 also. The Baboons of Batchianhave not passed to Gilolo, a mucli 

 larger island, only separated from it by a channel ten miles wide, 

 and in one part almost blocked up witb small islands. 



Now looking at these phenomena of distribution, and especially 

 at those presented by tbe fauna of Celebes, it appears to me that 

 a much exaggerated effect, in producing the present distribution 

 of animals, has been imputed to the accidental transmission of 

 individuals across intervening seas ; for we have here as it were 

 a test or standard by which we may measure the possible effect 

 due to these causes, and we find that, under conditions perhaps the 

 most favourable that exist on the globe, the percentage of species 

 derived from this source is extremely small. "When my researches 

 in the Archipelago are completed, I hope to be able to determine 

 with, some accuracy this numerical proportion in several cases ; but 

 in the mean time we will consider 20 per cent, as the probable 

 maximum for birds and mammals which in Celebes have been 

 derived from Borneo or Java. 



Let us now apply this standard to the case of Great Britain and 

 the Continent, in which the width of dividing sea and the extent 

 of opposing coasts are nearly the same, but in which the species 

 are almost all identical, — or to Ireland, more than 90 per cent, 

 of whose species are British, — and we shall at once see that no 

 theory of transmission across the present Straits is admissible, and 

 shall be compelled to resort to the idea of a very recent separation 

 (long since admitted), to account for these zoological phenomena. 



It is, however, to the oceanic islands that we consider the appli- 

 cation of this test of the most importance. Let any one try to 

 realize the comparative facilities for the transmission of organized 

 beings across the Strait of Macassar from Borneo to Celebes, and 

 from South Europe or North Africa to the island of Madeira, at 

 least four times the distance, and a mere point in the ocean, and 

 he would probably consider that in a given period a hundred cases 

 of transmission would be more likely to occur in the former case 



