Other evidences, however, drawn from other sources, as we shall see hereafter, are not wholly wanting. CHAPTER V. PROOFS FROM HOMOLOGIES OF THE VERTEBRATE SKELETON. The proposition to be established here is, that all vertebrates have not only a common general plan of structure, but an essential identity even in detail, although this identity is obscured by adaptive modifications. We will try to show first a common general plan, and then, taking parts most familiar to the general reader, will show essential identity even in detail. =Common General Plan.=--1. All vertebrate animals, and none other, have an _internal_ jointed skeleton worked by muscles on the _outside_. As we shall see hereafter, the relation of skeleton and muscle in arthropods is exactly the reverse. 2. In all vertebrates, and in none other, the axis of this skeleton is a jointed backbone (vertebral column) inclosing and protecting the nervous centers (cerebro-spinal axis). These, therefore, may well be called back-boned animals. 3. All vertebrates, and none other, have a number of their anterior vertebral joints enlarged and consolidated into a box to form the skull,[22] in order to inclose and protect a similar enlargement of the nervous center, viz., the brain; and also usually, but not always, a number of posterior joints, enlarged and consolidated to form the pelvis, to serve as a firm support to the hind-limbs. 4. All vertebrates, and none other, have two cavities, inclosed and protected by the skeleton, viz., the neural cavity above, and the visceral or body cavity below, the vertebral column; so that a cross-section of the body is diagrammatically represented by Fig. 4. [Illustration: FIG. 4.--Diagram cross-section through the body of a vertebrate, showing the relation of skeleton to the cavities. _n_, neutral cavity; _v_, visceral cavity; _c_, centrum of vertebra. ] 5. All vertebrates, with few exceptions, and no other animals, have two and only two pair of limbs. The exceptions are of two kinds, viz.: _a_, some lowest fishes, amphioxus and lampreys, which probably represent the vertebrate condition before limbs were acquired; and _b_, degenerate forms like snakes and some lizards, which have lost their limbs by disuse. So much concerns the general plan of skeletal structures, and is strongly suggestive of--in fact, is inexplicable without--common origin. But much more remains which is not only suggestive, but demonstrative of such origin. By extensive comparison in the taxonomic and ontogenic series, the whole vertebrate structure in all its details in different animals may be shown to be modifications one of another. Sometimes a piece is enlarged, sometimes diminished, or even becomes obsolete; sometimes several pieces are consolidated into one; but, in spite of all these obscurations, corresponding parts may usually be made out. This is the main subject of this chapter.