another certain perception of resemblance in one other certain mind had _cuticular skeleton_ for its general idea, and as yet another perception of resemblance in yet other certain minds had _red blood_ for its general idea. As shown by the disparities which obtain among the section which the scheme serves to separate from the others, it no more determines rank or standing than that greatly more ancient scheme of classification into “ring-streaked and spotted,” which served to distinguish the flocks of the patriarch Jacob from those of Laban his father-in-law, but which did not distinguish goats from sheep, nor sheep from cattle. The effect of introducing, after this manner, generalizations made altogether irrespective of _rank_, and avowedly without reference to it, into what are inherently and specifically _questions of rank_, admits of a simple illustration. Let us suppose that it was not with the standing of the Silurian Placoids that we had to deal, but with that of the _mammals_ of the recent period, including the _quadrumana_, and even the _bimana_, and that we had ventured to describe them, in the words of the Edinburgh Reviewer, as “the very highest types of their class.” What would be thought of the reasoner who, in challenging the justice of the estimate, would argue that these creatures, men as well as monkeys, belonged simply to that division of red-blooded animals which includes, with the bimana and quadrumana, the frog, the gudgeon, and the _earthworm_?—a division, he might add, “which, when details of organization are regarded, stretches farther, both downward and upward,” than that division of the white-blooded animals to which the crab, the spider, the cuttle-fish, and the dragon fly belong; “so that, looking at one extremity, any one is as much entitled to call the red-blooded animals the lowest division, as any other, looking at another extremity, is to call them the highest division, of animals.” What, it might well be asked in reply, has the earthworm, with its red-blood to do in a question respecting the place and standing of the bimana? Or what, in the parallel case, have the _Suctorii_—the worms of Linnæus—to do in a question respecting the place and standing of the real Placoids? True it is that, according to one principle of classification, now grown somewhat obsolete, men and earthworms are equally red-blooded animals; true it is that, according to another principle of classification, the Placoids of Agassiz and the cartilaginous worms of Linnæus are equally _Chondropterygii_. The bimana and the earthworm have their red blood in common; the glutinous hag and the true Placoids have as certainly their internal cartilage in common; and if the fact of the red blood of the worm lowers in no degree the rank of the bimana, then, on the same principle, the fact of the internal cartilage of the glutinous hag cannot possibly detract from the standing of the true Placoid. In both cases they are creatures that entirely