180 
MR. A. R. WALLACE OK THE ZOOLOGICAL 
tribution of the genus Paradisea , two species of which (the com- 
mon Birds of Paradise) are found only in New Guinea and the 
islands of Aru, Mysol, AVaigiou, and Jobie, all of which arc 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 Batchian have not passed to Gilolo, a much 
larger island, only separated from it by a channel ten miles wide, 
and in one part almost blocked up with small islands. 
Now looking at these phenomena of distribution, and especially 
at those presented by the 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 
