CHAP. XXI.] INSECTS. 503 



best marked affinities between the regions are those between 

 the Nearctic and Palsearctic, — the Oriental and Australian, 

 — the Australian and Neotropical, — which appear to be about 

 equal in each case. Next comes that between the Ethiopian 

 and Oriental on the one side, and the Ethiopian and Neotropical 

 on the other, which also appear about equal. Then follows that 

 between the Nearctic and Neotropical regions ; and lastly, and far 

 the least marked, that between the North Temperate and South 

 Temperate regions. That the relation between the Ethiopian 

 and Neotropical region should be so comparatively well marked, 

 is unexpected ; but we must consider that in such a comparison 

 as the present, we probably get the result, not of any recent 

 changes or intermigrations, but of all the long series of changes 

 and opportunities of migration that have occurred during many 

 geological epochs, — probably during the whole of the Tertiary 

 period, perhaps extending far back into the Secondary age. 



It appears evident that Insects exhibit in a very marked 

 degree in their actual distribution, the influence both of very 

 ancient and very modern conditions of the earth's surface. The 

 effects of the ancient geographical features of the earth, are to be 

 traced, in the large number of cases of discontinuous and widely 

 scattered groups which we meet with in almost every family, 

 and which, to some extent, obscure the broader features of distri- 

 bution due to the period during which the barriers which divide 

 the several primary regions have continued to exist. And this, 

 which we may consider as the normal distribution, is still 

 further obscured in those cases where the barriers between 

 existing regions are of such a nature as to admit of the free 

 passage of insects or their larva in a variety of ways, and (what 

 is perhaps of more importance) in which the physical features 

 on both sides of the barrier are so nearly identical, as to admit 

 of the ready establishment of such immigrants as may occasion- 

 ally arrive. These conditions concur, for some families of insects, 

 in the case of the Oriental and Australian portions of the Malay 

 Archipelago: and it is there that the normal distribution has 

 been sometimes greatly obscured, but never, as we have suffi- 

 ciently shown, by any means obliterated. 



Vol. XL— 33 



