=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. [Illustration: 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.) ] [Illustration: FIGS. 10-13.--10. Fore-limb of bat. 11. Bird. 12. Archæopteryx. 13. Pterodactyl. (Lettered as in previous figures; grouped from various sources.) ] 2. The coracoid (_c_), it is seen, is a small, beak-like process of the blade-bone (scapula) in man and mammals; but in birds (Fig. 11) and reptiles (Figs. 14, 18) it is a separate bone as large as the blade-bone itself, jointed with the latter at the shoulder and with the breast-bone (sternum) in front, thus making together a strong shoulder-girdle for the attachment of the fore-limb. This was undoubtedly the condition in the original or earliest walking animal, viz., reptiles. It was inherited and retained by birds, because necessary for powerful action of the wings in flight. In mammals it gradually dwindled and became united with the blade-bone as a process. In one mammal, the lowest and most reptilian living--the ornithorhynchus--the coracoid is much like that of reptiles--a large, flat bone, separated from the blade-bone and articulated with the breast-bone. It is a significant fact that, in the mammalian embryo, it is first developed as a separate bone and afterward united with the scapula. [Illustration: FIGS. 14-17.--14. Fore-limb of turtle. 15. Mole. 16. Whale. 17. Fish.]