Touching the two-fold question, of the _local engagement_ of this Atlantic district with specific centres of diffusion, and the _extreme slowness of their diffusive progress_, much instruction may be derived from a contemplation of the conchological statistics. Porto Santo, for instance, is a very small island (not more than seven miles in length), yet the number of endemic species which it includes is so perfectly astounding that it may be appropriately termed a _generic area of radiation_. Nor does this primaeval excess of its aboriginal beings strike us more forcibly than does the utter quiescence (if I may so express it) which has been going on amongst them since the remote era of their birth. Although a few have apparently died out[64] since that epoch, consequent perhaps on the change of level and diminished range which took place during the process of subsidence; we are amazed to find that certain species which are now limited to particular spots (even whilst unopposed by physical barriers) have been absolutely peculiar to them from the first,--or, in other words, that, whilst the fossil deposits extend throughout the lower regions of the island, far and wide, it is only in those respective portions of the beds which join on to the present "habitats" that the fossil homologues of several of the species are to be met with. The _H. Wollastoni_ is eminently a case in point. That most interesting of the Madeiran mollusks was first detected by myself on the southern ascent of the Pico de Conseilho, of Porto Santo, April 22, 1849; and the subsequent explorations of the Rev. R. T. Lowe, in conjunction with my own, have, I think, satisfactorily proved that it occurs nowhere else except upon that single slope. Throughout the large expanse of calcareous incrustations which are spread over the island elsewhere, and on the adjoining Ilheo de Baixo, all of which teem with shells, I think I may assert, without fear of contradiction, that the _H. Wollastoni_ does not so much as exist. Yet at the Zimbral d'Areia, which the Pico de Conseilho directly overhangs,--a rich tract for these fossil remains,--as well as in the muddy composition of a cliff near at hand, it literally abounds. In like manner, we might recall many others which are peculiar, _recent and fossil_, to the self-same precincts. Such, for example, are the _H. calculus_ and _commixta_, which swarm on the summit of the Ilheo de Baixo, in both states. The _H. attrita_, again, is the Pico d'Anna Ferreira modification of the _H. polymorpha_; and it is only in the beds towards the base of that mountain that its fossil homologue is found. But what do these facts indicate? Surely they tell us plainly of what we have already so often insisted upon,--namely, the redundancy of this once continuous land with specific foci of its own, and the sluggish or sedentary nature of those primaeval radiating forms.