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, Sardinia, and North-western Africa, and _vice versâ_. The Balearic Islands were then connected with Spain; and we find there many curious survivals which have long ago become extinct on the mainland.