also belong to this fauna, as also the
Earthworms _Allolobophora veneta_ and _A. Georgii_, and the Millipede
_Polydesmus gallicus_.

It will be evident to every one from these few instances of Lusitanian
species, that somewhere in South-western Europe and North-western
Africa, and also, perhaps, in a larger now submerged western land-area,
there existed an active centre of development, from which animals spread
in all directions.

If the presence of _Platyarthrus_ in North-west Africa proves that the
Straits of Gibraltar had come into existence after its southward
migration, it also suggests that the ancestral home of this woodlouse
was in the Spanish peninsula. Whether this supposition is correct or
not, does not affect the Straits of Gibraltar problem, for in a
migration northward into Spain from Morocco a land-connection would be
equally necessary. Almost every group of vertebrates and invertebrates
furnishes instances of species which must have crossed the Straits on
dry land. Many naturalists have come to this conclusion, and have
clearly expressed their views on the subject. At the commencement of the
present period, says Mr. Bourguignat (p. 354), the north of Africa was a
peninsula of Spain, the Straits of Gibraltar did not exist, and the
Mediterranean communicated by the Sahara with the Atlantic.

The faunas of North-west Africa and the south-western portion of our
continent are so closely related, that an uninterrupted intercourse by
land must have existed for a very long period. The Mediterranean,
however, throughout the Tertiary period--at any rate since miocene
times--must have had almost constant communication with the Atlantic.
According to Professor Suess, this was the case. The Atlantic was joined
with the Mediterranean across the valley of the Guadalquivir during the
Miocene Epoch, so that Andalusia must have belonged to North Africa in
those days. The Straits of Gibraltar are supposed to have been formed in
the next epoch. I have already expressed my disagreement with that
theory from a zoogeographical point of view. The old Guadalquivir
connection probably persisted much longer,--though interrupted by
temporary periods of a partial retreat--so as to uncover sufficient land
to allow of an interchange during miocene as well as pliocene times
between the European and North African faunas. It is in this way,
perhaps, that some of the members of the Alpine fauna have reached Spain
by way of Corsica, Sar