being struck by the fact that the species
after their entry into Europe evidently did not all follow the same path
during their westward advance. We have seen that a good many seem to
have travelled either due west or north-west on entering our continent
from Asia Minor. They may now perhaps be found in Greece, Southern
Italy, Algiers, and Spain, also probably on some of the intervening
islands in the Greek Archipelago, in Sicily, Sardinia, and Corsica, or
they may have travelled north-east and occur in the Alps. This
distribution indicates undoubtedly, as I have already set forth in
another memoir (_c_, p. 459), that land extended from Asia Minor across
Greece to Southern Italy, that the latter again was disconnected with
Central Italy, but united with Sicily, Sardinia, and Tunis, and that the
Straits of Gibraltar did not exist at the time when these species
migrated westward. Some species are only to be found as far west as
Southern Italy, while others occur in Central and Northern Europe,
scarcely in the South, and not at all in the larger Mediterranean
islands or in North Africa. This appears to me to indicate that the late
comers from the east found that geographical changes had taken place in
Southern Europe which prevented them from following the same track as
the older immigrants. They were now obliged to turn directly northward
and then westward. It may be asked, why should not the earlier migrants
have taken the same route? This question will be answered immediately.
Meanwhile it should be clearly understood that there probably was an
older and a newer migration from the east. The Oriental genera--from
whose general range we know that they must be very ancient indeed, such
as _Mantis_ and _Bacillus_--are almost invariably confined to Southern
Europe. There they are frequently found on some of the Mediterranean
islands. The earlier migrants therefore went westward and the later ones
northward.

Let us now inquire a little into the reasons why such different courses
were pursued by the migrants--why the Oriental migration divided into
two streams, an older and a newer.

During early Tertiary times, and probably throughout the Miocene and
Pliocene Epochs, the AEgean Sea did not exist. From the island of Crete
to the Peloponnesus, and from Asia Minor to Thessaly and Macedonia,
stretched a vast and fertile plain dotted over with numerous freshwater
lakes. Gradually the sea encroached upon this land from the south,