Here another geological consideration comes in to help on this inference.  The species of the later tertiary period for the most part not only resembled those of our days, many of them so closely as to suggest an absolute continuity, but, also occupied in general the same regions that their relatives occupy now.  The same may be said, though less specially, of the earlier tertiary and of the later secondary; but there is less and less localization of forms as we recede, yet some localization even in palaeozoic times.  While in the secondary period one is struck with the similarity of forms and the identity of many of the species which flourished apparently at the same time in all or in the most widely separated parts of the world, in the tertiary epoch, on the contrary, along with the increasing specialization of climates and their approximation to the present state, we find abundant evidence of increasing localization of orders, genera, and species; and this localization strikingly accords with the present geographical distribution of the same groups of species.  Where the imputed forefathers lived, their relatives and supposed descendants now flourish.  All the actual classes of the animal and vegetable kingdoms were represented in the tertiary faunas and floras, and in nearly the same proportions and the same diversities as at present.  The faunas of what is now Europe, Asia, America, and Australia differed from each other much as they now differ:  in fact,—­according to Adolphe Brongniart, whose statements we here condense,[a]—­the inhabitants of these different regions appear for the most part to have acquired, before the close of the tertiary period, the characters which essentially distinguish their existing faunas.  The eastern continent had then, as now, its great pachyderms, elephants, rhinoceros, and hippopotamus; South America its armadillos, sloths, and ant-eaters; Australia a crowd of marsupials; and the very strange birds of New Zealand had predecessors of similar strangeness.  Everywhere the same geographical distribution as now, with a difference in the particular area, as respects the northern portion of the continents, answering to a warmer climate then than ours, such as allowed species of hippopotamus, rhinoceros, and elephant to range even to the regions now inhabited by the reindeer and the musk-ox, and with the serious disturbing intervention of the glacial period within a comparatively recent time.  Let it be noted, also, that those tertiary species which have continued with little change down to our days are the marine animals of the lower grades, especially mollusca.  Their low organization, moderate sensibility, and the simple conditions of an existence in a medium like the ocean, not subject to great variation and incapable of sudden change, may well account for their continuance; while, on the other hand, the more intense, however gradual, climatic vicissitudes on land, which have driven all tropical and sub-tropical forms out of the higher latitudes and assigned to them their actual limits, would be almost sure to extinguish such huge and unwieldy animals as mastodons, mammoths, and the like, whose power of enduring altered circumstances must have been small.