Euslera Pieridce. 305 



conditions are almost identical tliroughoiit, it is not surprising 

 that an original diversity should have been reduced to an approxi- 

 mate uniformity of type in the two regions. 



If this is the true explanation of the phenomenon, we ought 

 to find the distinction of the two regions more marked in those 

 orders and families where there is less probability of involuntary 

 diflTusion. The butterflies are almost entirely free from the 

 chance of being carried from island to island by floating timber, 

 but they are at all times subject to be blown great distances 

 I)y storms and by the strong and clianging monsoons. This last 

 cause of diffusion is perhaps quite as powerful as the former 

 one, but it is probable that these insects are not able to establish 

 themselves in new countries with so much certainty as the 

 Calcoptera, (as might be expected from the dependence of many 

 of the species on particular plants, which they miglit not find in 

 the place to whicli they were carried) ; for we find many instances 

 in which genera and sections of genera are strictly limited to 

 one-half of the Archipelago or the other. For example, the 

 genera Mynes and IJj/pociila are confined strictly to the Aus- 

 tralian region, and Drusilla is only represented out of that 

 region by one species in Java. To this region also belong the 

 genus Elodlna, the Erecllieus and Priamus groups of Pajnlio, the 

 Rachd group of Vieris, the Celestina group of Tachyris, and 

 the Mysis and Philyra group of TItyca, On the otiier hand, 

 Prioneris, Dcrcas and T/iauiuaiitis range over the whole Indian 

 region, but do not cross the dividing line ; while Thest'ins, the 

 Coronis group of Pieris, and the genera Euripus, Clerome, 

 yJmnt/uisia, ZciixidJa and Discophora, only pass over the boundary 

 into Celebes, or along the cl<)se!y linked chain of islands east 

 of Java to rimor. In most of these cases, the same groups that 

 inhabit the IMoluccas and New Guinea are found also in Australia, 

 and in some of the islands of the Pacific ; while, notwithstanding 

 the similarity of physical conditions, and the narrowness of the 

 dividing seas, tliey do not |)ass into Java, Borneo, or the other 

 western islands. These facts go to prove, that notwithstanding 

 the causes favourable to migration and intermixture in the case 

 of insects, yet the isolation of groups dependent on ancient and 

 permanent features in the physical geography of the country 

 may still be very clearly traced ; and if so, the primary zoo- 

 li)gical divisions of the earth, founded on the facts of distribution 

 presented by the higher animals, will be found equally applicable 

 to the objects of the Entomologist's study, since it will enable hiin 



