[28] It will scarce be urged against the degradation theory, that those races which, tried by the tests of defect or misplacement of parts, we deem degraded, are not less fitted for carrying on what in their own little spheres is the proper business of life, than the non-degraded orders and families. The objection is, however, a possible one, and one which a single remark may serve to obviate. It is certainly true that the degraded families _are_ thoroughly fitted for the performance of all the work given them to do. They greatly increase when placed in favorable circumstances, and, when vigorous and thriving, enjoy existence. But then the same may be said of all animals, without reference to their place in the scale;—the mollusc is as thoroughly adapted to its circumstances and as fitted to accomplish the end proper to its being, as the mammiferous quadruped, and the mammiferous quadruped as man himself; but the fact of perfect adaptation in no degree invalidates the other not less certain fact of difference of rank, nor proves that the mollusc is equal to the quadruped, or the quadruped to man. And, of course, the remark equally bears on the _reduced_ as on the _unelevated_,—on lowness of place when a result of degradation in races pertaining to a higher division of animals, as on lowness of place when a result of the humble standing of the division to which the races belong. [29] The vertebral column in the genus _Diplopterus_ ran, as in the placoid genus _Scyllium_, nearly through the middle of the caudal fin. [30] In the following diagram a few simple lines serve to exhibit the progress of degradation. Fig. _a_ represents the symmetrical Placoids of the Silurian period, consisting of head, neck, body, tail, fore limbs and hinder limbs; fig. _b_ represents those heterocercal Ganoids of the Old Red Sandstone, Coal Measures, and Permian System, in which the neck is extinguished, and the fore limbs stuck on to the occiput; fig. _c_, those homocercal Ganoids of the Trias Lias, Oolite, and Wealden, whose tails spread out into broad terminal processes, without homologue in the higher animals; fig. _d_, those Acanthopterygii of the Chalk that, in addition to the non-homological processes, have both fore limbs and hinder limbs stuck round the head; while fig. _e_ represents the asymmetrical Platessa, of the same period, with one of its eyes in the middle of its head, and the other thrust out to the side. [Illustration]