In the same way we might pursue the _H. erubescens_, and show that in the sylvan regions, and on the low barren Ponta Sao Lourenco of Madeira, on the Pico de Facho of Porto Santo, on the Ilheo Chao, on the Central Dezerta, and on the Bugio (where it attains a gigantic size), it has its distinct and permanent phases,--the evident results of isolation, and other topographical influences, since the subsidence of the intervening tracts. And in like manner, the _Clausilia deltostoma_ is universal throughout the Madeiran Archipelago,--displaying, however, in Porto Santo a fixed and strongly ribbed state, peculiar to that island. Thus, if the examples which we previously cited tend to establish the extreme slowness of the migratory movements of the terrestrial mollusca across this former continent, the present ones (which refer to a few exceptional species of quicker self-diffusive powers) will show, no less than the _insects_ to which I have lately called attention, that where sufficient areas had been overspread (before the periods of subsidence) for the creatures to have reached what now constitute the various islands of the cluster, we at once detect traces of this fact, through their more or less altered aspects,--the result of isolation, and diminished range, during the enormous interval which has elapsed since the successive convulsions which caused the partial destruction of this Atlantic province were brought to a close. To return, however, to the insects, after this long conchological digression,--I need not multiply evidence, in corroboration of my theory. Enough has been said to render intelligible the idea which I wished to convey, concerning the _general direction_ of the migratory current over that ancient tract, and the _extreme slowness of its progress_,--the former of which I consider probable from the north-easterly course in which creatures _generically identical_ were, if we may so express it, "given-off;" whilst the circumstance of their being for the most part _specifically dissimilar_ (or, in other words, of the islands harbouring, many of them, species which are endemic) would seem as it were to establish the latter.