a fish closely related to _particular_ reptiles) might occur, namely from a few descendants of those species, which at a very early period branched out from a common parent-stock and so formed the two orders or groups, having survived, in nearly their original state, to the present time. {451} A similar discussion occurs in the _Origin_, Ed. i. p. 427, vi. p. 582. {452} _Puffinuria berardi_, see _Origin_, Ed. i. p. 184, vi. p. 221. {453} _Origin_, Ed. i. p. 430, vi. p. 591. Finally, then, we see that all the leading facts in the affinities and classification of organic beings can be explained on the theory of the natural system being simply a genealogical one. The similarity of the principles in classifying domestic varieties and true species, both those living and extinct, is at once explained; the rules followed and difficulties met with being the same. The existence of genera, families, orders, &c., and their mutual relations, naturally ensues from extinction going on at all periods amongst the diverging descendants of a common stock. These terms of affinity, relations, families, adaptive characters, &c., which naturalists cannot avoid using, though metaphorically, cease being so, and are full of plain signification. CHAPTER VIII UNITY OF TYPE IN THE GREAT CLASSES; AND MORPHOLOGICAL STRUCTURES _Unity of Type_{454}. {454} _Origin_, Ed. i. p. 434, vi. p. 595. Ch. VIII corresponds to a section of Ch. XIII in the _Origin_, Ed. i. Scarcely anything is more wonderful or has been oftener insisted on than that the organic beings in each great class, though living in the most distant climes and at periods immensely remote, though fitted to widely different ends in the economy of nature, yet all in their internal structure evince an obvious uniformity. What, for instance, is more wonderful than that the hand to clasp, the foot or hoof to walk, the bat's wing to fly, the porpoise's fin{455} to swim, should all be built on the same plan? and that the bones in their position and number should be so similar that they can all be classed and called by the same names. Occasionally some of the bones are merely represented by an apparently useless, smooth style, or are soldered closely to other bones, but the unity of type is not by this destroyed, and hardly rendered less clear. We see in this fact some deep bond of union between the organic beings of the same great classes--to illustrate which is the object and foundation of the natural system. The perception of this bond, I may add, is the evident cause that naturalists make an ill-defined distinction between true and adaptive affinities. {455} _Origin_, Ed. i. p. 434, vi. p. 596. In the _Origin_, Ed. i. these examples occur under the heading _Morphology_; the author does not there draw much distinction between this heading and that of _Unity of Type_. _Morphology._