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.
Special Homology of Vertebrate Limbs.—It would lead us much too far into unfamiliar technicalities to take up the whole skeleton. We select the limbs, both because their general structure is more familiar, and because in them the two fundamental ideas of essential identity and of adaptive modification are both admirably illustrated. The reason of this is, that it is by the limbs that the organism chiefly reacts on the environment, and is modified by it.
Fore-limbs.—In the accompanying figures ([Figs. 5–18]) we have represented, side by side, the fore-limbs of many vertebrates, taken from all the classes—mammals, birds, reptiles, and fishes. For convenience of comparison, the corresponding parts are similarly lettered in all. Also, in order to identify easily certain important corresponding segments, we have drawn through them a continuous dotted line. In man, nearly all the parts are present, and his limbs, therefore, may be taken as a term of comparison; for man’s structure, except his brain, is far less modified than that of many animals.
Note, then, the following points: 1. The collar-bone (clavicle) is associated with wide separation of the shoulders, and the free use of the fore-limb for prehension or for flight, but is gradually lost in proportion as the fore-limb is brought nearer together and used for support, because it is no longer wanted. I say gradually, for all the steps of the passing away may be found. The useless rudimentary condition is not uncommon.
Figs. 5–9.—5. Fore-limb of man. 6. Dog 7. Hog. 8. Sheep. 9. Horse. sc, scapula; c, coracoid; a, b, two bones of fore-arm. (Taken from various sources and grouped.)