It is of importance that not only the direct _bearing_, but also the actual _amount_, of the evidence in this case, should be fairly stated. So far as it extends, the testimony is clear; but it does not extend far. All the vertebrate remains yet detected in the Silurian System, if we except the debris of the Upper Ludlow bone-bed, might be sent through the Post-Office in a box scarcely twice the size of a copy of the “Vestiges.” The naturalist of an exploring party, who, in crossing some unknown lake, had looked down over the side of his canoe, and seen a few fish gliding through the obscure depths of the water, would be but indifferently qualified, from what he had witnessed, to write a history of _all_ its fish. Nor, were the some six or eight individuals of which he had caught a glimpse to be of small size, would it be legitimate for him to infer that only small-sized fish lived in the lake; though, were there to be some two or three large ones among them, he might safely affirm the contrary. Now, the evidence regarding the fishes of the Silurian formation very much resembles what that of the naturalist would be, in the supposed case, regarding the fishes of the unexplored lake; with, however, this difference, that as the deposits of the ancient system in which they occur have been examined for years in various parts of the world, and all its characteristic organisms, save the ichthyic ones, found in great abundance and fine keeping, we may conclude that the fish of the period were comparatively few. The palæontologist, so far as the question of number is involved, is in the circumstances, not of the naturalist who has only once crossed the unknown lake, but of the angler who, day after day, casts his line into some inland sea abounding in shell-fish and crustacea, and, after the lapse of months, can scarce detect a nibble, and, after the lapse of years, can reckon up all the fish which he has caught as considerably under a score. The existence of this great division of the animal kingdom, like that of the earlier reptiles during the Carboniferous period, did not form a prominent characteristic of those ages of the earth’s history in which they began to be.