CHAP. XXI.] 
INSECTS. 
503 
best marked affinities between the regions are those between 
the Nearctic and Palaearctic, — 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. 
