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48 ZOOLOGICAL GEOGRAPHY. [part hi.
number and variety of the tropical forms has given them tiie ad-
vantage. Thus we find that among the Lucanidse, Buprestidse, and
Longicorns, the northern element is hardly perceptible. Most of
these are either purely Neotropical, or allied to Neotropical genera,
with the admixture, however, of a decided Australian element.
As in the case of the Amphibia and fresh-water fishes, the Aus-
tralian affinity, as shown by insects, is of two kinds, near and
remote. We have a few genera common to the two countries ;
but more commonly the genera are very distinct, and the affinity
is shown by the genera of both countries belonging to a group
peculiar to them, but which may be of very great age. In the
former case, we must impute some of the resemblance of the two
faunas to an actual interchange of forms within the epoch of
existing genera— a period of vast and unknown duration in the
class of insects ; while in the latter case, and perhaps also in
many of the former, it seems more in accordance with the whole
of the phenomena, to look upon most of the instances as
survivals, in the two southern temperate areas, of the rehcs of
groups which had once a much wider distribution. That this is
the true explanation, is suggested by the numerous cases of dis-
continuous and scattered distribution we have had to notice, in
which every part of the globe, without exception, is implicated ;
and there is a reason why these survivals should be rather more
frequent in Australia and temperate South America, inasmuch
as these two areas agree in the absence of a considerable number
of otherwise cosmopolitan vertebrate types, and are also in many
respects very similar in climatic and other physical conditions.
The preponderating influence of the organic over the physical
environment, as taught by Mr. Darwin, leads us to give most
weight to the first of the above-mentioned causes ; to which we
may also impute such undoubted cases of survival of ancient
types as the Centetidse of the Antilles and Madagascar—both
areas strikingly deficient in the higher vertebrate forms. The
probable mode and time of the cross migration between Australia
and South America, has been sufficiently discussed in our chapter
on the Australian region, when treating of the origin and affinities
of the New Zealand fauna.