in their descendants, but in restoring to the offspring the true limbs and eyes which the parents have lost. It is, however, not with a theory of hereditary degradation, but a hypothesis of gradual development, that I have at present to deal; and what I have to establish as proper to the present stage of my argument is, that this principle of degradation really exists, and that the history of its progress in creation bears directly against the assumption that the earlier vertebrata were of a lower type than the vertebrata of the same ichthyic class which exist now.[28] The progress of the ichthyic tail, as recorded in geologic history, corresponds with that of the ichthyic limbs. And as in the existing state of things we find fishes that _nearly_ represent, in this respect, all the great geologic periods,—I say _nearly_, not _fully_, for I am acquainted with no fish adequately representative of the period of the Old Red Sandstone,—it may be well to cast a glance over the _contemporary_ series, as illustrative of the _consecutive_ one. In those Placoids of the shark family that to a large brain unite homological symmetry of organization, and represent the fishes of the first period, we find, as I have already shown, that the vertebræ gradually diminish in the caudal division of the column, until they terminate in a point,—a circumstance in which they resemble not merely the betailed reptiles, but also all the higher mammiferous quadrupeds, and even man himself. And it is this peculiarity, stamped upon the less destructible portions of the framework of the tail,—vertebræ and processes,—rather than the one-sided or heterocercal form of the surrounding fin, composed of but a mucoidal substance, that constitutes its grand characteristic; seeing that in some Placoid genera, such as _Scyllium Stellare_, the terminal portion of the fin is scarce less largely developed above than below, and that in others, as in most of the Ray family, the under lobe of the fin is wholly wanting. In the sturgeon,—one of the few Ganoids of the present time,—we become sensible of a peculiar modification in this heterocercal type of tail: the lower lobe is, we find, composed, as in _Spinax_ and _Scyllium_, of rays exclusively; while through the centre of the upper lobe there runs an acutely angular patch of lozenge-shaped plates, like that which runs through the centre of the double fins of _Dipterus_ and the Cœlacanths. But while in the sharks the gradually diminishing vertebræ stand out in bold relief, and form the thickest portion of the tail, that which represents them in the sturgeon (the angular patch) is slim and thin,—slimmer in the middle than even at the sides;—in part a consequence, no doubt, of the want, in this fish, of solid vertebræ, but a consequence also of the extreme attenuation of the nervous cord, in its prolongation into the lobe of the fin. Further, the rays of the tail—its peculiarly ichthyic portion, which are purely