I cannot close this chapter without referring to the active distributional centre--or I might say, centre of origin--of species situated in South-eastern Europe. No group of animals is more instructive in elucidating the paths of migration from this centre than the terrestrial mollusca. Wherever the original home of the genus _Clausilia_ may have been in early Tertiary times, it is certain that the most active centre of origin is now, and has been for a considerable time past, in South-eastern Europe. One of the earliest migrants from that modern centre of this interesting genus is _Clausilia bidentata_, which is the only species found in Southern Spain, and one of the two met with in Ireland, and which has been observed in high altitudes in the Alps and in Scandinavia. As we go eastward from Western Europe the number of species of _Clausilia_, as we have seen, increases until we reach a maximum in the Balkan peninsula and the region of the Caucasus. _Limax_, _Agriolimax_, and _Amalia_, three genera of slugs, likewise appear to have originated in the same region and spread over Europe from there. Some species like _Limax maximus_ and _L. marginatus_ are very ancient, and probably commenced their wanderings in early Tertiary times. In this manner many animals of European origin have joined the Oriental migrants in their westward and also in their later northward travels. In a similar way species of plants and animals of Alpine origin might have joined these migrants in their northward course, and it is only when we come to carefully analyse the constituent parts of all these members which have come to us in England from the south, that we realise the complexity of their origin. Finally, even the Siberian migrants mingled with the later Oriental ones, and in some cases the decision as to whether a certain species belongs to the former or to the latter migration becomes a matter of great difficulty. SUMMARY OF CHAPTER VI.