PDF - Wallace Online

CHAP. XIV.] THE NEOTROPICAL REGION. 45
The Palsearctic affinity of the South Temperate Carabida^ may
be readily understood, if we bear in mind the great antiquity of the
group, and the known long persistence of generic and specific
forms of Coleoptera ;
the facility with which they may be trans-
ported to great distances by gales and hurricanes, either on land
or over the sea ; and, therefore, the probability that suitable
stations would be rapidly occupied by species already adapted
to them, to the exclusion of those of the adjacent tracts which
had been specialised under different conditions. If, for example,
we carry ourselves back to the time when the Andes had only
risen to half their present altitude, and Patagonia had not
emerged from the ocean (an epoch not very remote geologically),
we should find nearly all the Carabidse of South America,
adapted to a warm, and probably forest- covered country. If,
then, a further considerable elevation of the laud took place, a
large temperate and cold area would be formed, without any
suitable insect inhabitants. During the necessarily slow pro-
cess of elevation, many of the tropical Carabidse would spread
upwards, and some would become adapted to the new conditions
while the majority would probably only maintain themselves by
continued fresh immigrations. But, as the mountains rose,
another set of organisms would make their way along the
highest ridges. The abundance and variety of the North
Temperate Carabidse, and their complete adaptation to a life on
barren plains and rock-strewn mountains, would enable them
rapidly to extend into any newly-raised land suitable to them;
and thus the whole range of the Eocky Mountains and Andes
would obtain a population of northern forms, which would over-
flow into Patagonia, and there, finding no competitors, would
develope into a variety of modified groups. This migration.was
no doubt effected mainly, during successive glacial epochs, when
the mountain-range of the Isthmus of Panama, if moderately
increased in height, might become adapted for the passage of
northern forms, while storms would often carry insects I'rom
peak to peak over intervening forest lowlands or narrow
straits of sea. If this is the true explanation, we ought to find
no such preponderant northern element in groups which