Here, then, the author of the “Vestiges” palpably founds on a large development of brain in the dolphin, and on the manifestation of a correspondingly high order of instincts,—and this altogether irrespective of the structure or composition of the creature’s internal skeleton. The substance to which he looks as all-important in the case is _brain_, not _bone_. For were he to estimate the standing of the dolphin, not by its brain, but by its skeleton, he would have to assign to it a place, not only _not_ in advance of its brethren the _mammalia_ of the sea, but even in the rear of the _reptiles_ of the sea, the marine tortoises, or turtles,—and scarce more than abreast of the osseous fishes. “Fishes,” says Professor Owen, in his “Lectures on the Vertebrate Animals,” “have the least proportion of earthy matter in their bones; birds the largest. The mammalia, especially the active, predatory species, have more earth, or harder bones, than reptiles. In each class, however, there are differences in the density of bone among its several members. For example, in the fresh-water fishes, the bones are lighter, and retain more animal matter, than in those which swim in the denser sea. And in the _dolphin_, a warm-blooded marine animal, they differ little in this respect from those of the sea-fish.” Such being the fact, it is surely but fair to inquire of the author of the “Vestiges,” why he should determine the rank and standing of the _Delphinidæ_ according to one set of principles, and the rank and standing of the Placoids according to another and entirely different set? If the _Delphinidæ_ are to be placed high in the scale, notwithstanding the softness of their skeletons, simply because their brains are large, why are the Placoids to be placed low in the scale, notwithstanding the largeness of their brains, simply because their skeletons are soft? It is not too much to demand, that on the principle which he himself recognizes as just, he should either degrade the dolphin or elevate the Placoid. For it is altogether inadmissible that he should reason on one set of laws when the exigencies of his hypothesis require that creatures with soft skeletons should be raised in the scale, and on another and entirely different set when its necessities demand that they should be depressed.