ACULEATE HTMENOPTERA AND lOHNEUMONIDiE. 301 



species of adjacent islands, we find similar results. For example, 

 of 109 ants found in Borneo and the Malay peninsula, only 

 12 are common to both, or 11 per cent. ; while of the rest of the 

 Hymenoptera, 198 in number, 33 are common, or near 17 per 

 cent. This fact is important, because we learn from it that 

 genera and species are distributed in the same manner, the want 

 of the power of flight leading to a more restricted range of both. 

 In the case of species this is very intelligible, on the simple prin- 

 ciple that the present distribution of animals is the result of na- 

 tural causes ; but when we find the same law hold for genera, it 

 altogether ceases to be intelligible, unless we suppose species to 

 undergo modification, so that the individuals of a species become 

 in time the species of a genus, in which case their distribution will 

 of course be regulated in a similar manner. The fact therefore 

 that the power of flight affects the distribution of genera in the 

 same manner as species, is a direct argument in favour of the for- 

 mation of the one from the other by a natural process of modi- 

 fication. 



In order to ascertain if the Hymenoptera show plainly the 

 division of the archipelago into two great regions, I will compare 

 the species of Borneo with those of the Malay peninsula on the 

 one hand, and with those of Celebes on the other. On looking 

 at the map, it will be at once seen that the facilities for passing 

 from Borneo to Celebes are much greaber than from Borneo to Ma- 

 lacca : yet, in the former case, out of a total of 479 Hymenoptera 

 collected by me in the two islands, only 27 were found in both, 

 equal to less than six per cent ; in the latter case, out of a total 

 of 307 species 45 were common to both, or about fifteen per 

 cent., — plainly indicating that some other cause than the present 

 proximity and facilities for migration has determined the exist- 

 ing distribution, the cause being, as I believe, that Borneo has 

 been recently connected with Malacca, but has never been united to 

 Celebes. The distinctness of the Hymenoptera of the two regions 

 of the archipelago, however, is much greater than is shown by the 

 mere statement of the number of species and genera peculiar to 

 each, since there are many other genera which have a maximum 

 in one region and give a character to its entomology, while, be- 

 cause a few straggling species have passed into the other region, 

 they do not appear as peculiar groups in either. Thus Cremato- 

 gaster, Atta, Cataulacus, Elis, Ammophila, Ampulcx, Tachytes, 

 Halictus, and Ceratina are characteristic of the Indian region, 



lilNN. JOURN. — ZOOLOGY, TOL. XT. 22 



