As an example of a genus which has its centre of distribution in South-Western Europe we might take that to which our common brown garden slug belongs, viz., _Arion_. Dr. Simroth, who was the first to point out that the species of _Arion_ had spread over our continent from South-Western Europe (p. 5), is inclined to the belief that the _Arionidæ_ had originated on the old land-bridge between Europe and North America, which is generally known by the name of "Atlantis." From this a branch went westward to the New World and another eastward as far as Southern Asia, but _Arion_ and a number of other genera are more or less confined to South-Western Europe. Only a few species of _Arion_ have a wide range in Europe, one of them, _A. subfuscus_, crossing the borders of our continent into Siberia. In the British Islands and in Western Germany, which are about equi-distant from the supposed creative centre of the genus, there are found five species. In France six or seven species are met with, and in Spain and Portugal about ten. Towards the east, _Arions_ diminish in number. This genus, therefore, forms part of a migration which I have designated as "Lusitanian" from _Lusitania_, the name applied by the Romans to what we now call Portugal. Another genus of slugs, _Geomalacus_, is interesting from the fact that one species occurs in the British Islands, being otherwise confined to the Lusitanian province. Parmacella, a slug-like animal bearing a tiny shell at the extremity of its tail, has probably likewise had its origin in this part of Europe. All this, however, will be more fully referred to in the seventh chapter, which deals with the Lusitanian fauna. As regards the Alpine centre of origin, Dr. Kobelt considers three groups of mollusca as especially characteristic of the Alps, viz., the sub-genus _Campylaea_ of the great and widely-spread genus _Helix_, and the genera _Pomatias_ and _Zonites_. The latter, which is not to be confounded with our British _Hyalinia_ (formerly united with _Zonites_), does not extend very far south or north of the Alps. There may be others too, which owe their origin to these mountains, but most of the terrestrial mollusca are exceedingly ancient, and many genera have existed long before the Alps had made their appearance above the surface of the early Tertiary seas. It should be remembered that _Hyalinia_ and _Pupa_, both British genera, are known from carboniferous deposits in forms which closely approach those living at the present day, and in these and a great number of other instances, it is quite impossible to determine the original home of the genus.