Turning to the _Mollusca_, the genera _Crania_, _Discina_, _Lingula_, have persisted from the Silurian epoch to the present day, with so little change, that very competent malacologists are sometimes puzzled to distinguish the ancient from the modern species. _Nautili_ have a like range, and the shell of the liassic _Loligo_ is similar to that of the "squid" of our own seas. Among the _Annulosa_, the carboniferous insects are in several cases referable to existing genera, as are the _Arachnida_, the highest group of which, the scorpions, is represented in the coal by a genus differing from its living congeners only in the disposition of its eyes. The vertebrate sub-kingdom furnishes many examples of the same kind. The _Ganoidei_ and _Elasmobranchii_ are known to have persisted from at least the middle of the Palæozoic epoch to our own times, without exhibiting a greater amount of deviation from the typical characters of these orders, than may be found within their limits at the present day. Among the _Reptilia_, the highest group, that of the _Crocodilia_, was represented at the beginning of the Mesozoic epoch, if not earlier, by species identical in the essential character of their organization with those now living, and presenting differences only in such points as the form of the articular faces of their vertebræ, in the extent to which the nasal passages are separated from the mouth by bone, and in the proportions of the limbs. Even such imperfect knowledge as we possess of the ancient mammalian fauna leads to the belief that certain of its types, such as that of the _Marsupialia_, have persisted with no greater change through as vast a lapse of time. It is difficult to comprehend the meaning of such facts as these, if we suppose that each species of animal and plant, or each great type of organization, was formed and placed upon the surface of the globe at long intervals by a distinct act of creative power; and it is well to recollect that such an assumption is as unsupported by tradition or revelation as it is opposed to the general analogy of Nature. If, on the other hand, we view "Persistent Types," in relation to that hypothesis which supposes the species of living beings living at any time to be the result of the gradual modification of pre-existing species--a hypothesis which though unproven, and sadly damaged by some of its supporters, is yet the only one to which physiology lends any countenance--their existence would seem to show, that the amount of modification which living beings have undergone during geological time is but very small in relation to the whole series of changes which they have suffered. In fact, palæontology and physical geology are in perfect harmony, and coincide in indicating that all we know of the conditions in our world during geological time, is but the last term of a vast and, so far as our present knowledge reaches, unrecorded progression. FOOTNOTES: