The fish-form governs that of the amphibian and of the reptile. The fins become limbs, which are variously modified. In some amphibians, as in the frog, the limbs become different in functions, the hind-limbs being adapted to a leaping motion, the fore-limbs to support, A differentiation of another kind takes place, eventually, in the reptile stock, the fore-limbs becoming organs of aerial support. They become wings, and we obtain the bird type, with fore-limbs adapted to aërial, hind-limbs to solid support.
These, however, are but aberrations from the general path of development, which is toward a continuance of the horizontal position of the fish-body: the fins become four similar supporting limbs.
Adaptation from this point takes one general direction, that of the reduction of weight to the lowest point consistent with the proper exercise of the nutritive functions, and the requisite strength of bone and vigor of muscle. The general shape attained by the body is governed by several mutually assisting or opposing functions. The requirement of speed needs that it shall be narrow and long, so as best to avoid the resistance of the air. But too great elongation would be a disadvantage, since the mid regions of the body would remain without support by the limbs, and could only sustain themselves by muscles acting to oppose gravity. These muscles would add to the weight, and would form additional consumers of force. The best form of the body, then, is one sufficiently narrowed to partly avoid aërial resistance, but not so elongated as to diminish its proper support upon the limbs. The chief aberrations from these requirements are those of the prone serpents, and, among mammals, of the weasel family, whose mode of life requires a very narrow body, so that digestive space can only be gained by its elongation. We may return here to the serpents to say that their elongation is probably due in great part to the same cause. Their mode of concealment and of motion requires great narrowness of body, so that space for the nutritive functions can only be gained by elongation. The requirement of narrowness is, in fact, so great that there is not sufficient room for the larger organs to be bilaterally reproduced, therefore respiration is confined to a single lung, the other being atrophied.
But, with a comparatively few exceptions, caused by highly specialized life-habits, the land vertebrates possess the general form requisite for the fullest adaptation to these three conditions—that of reduced resistance to motion, proper support, and the necessary room for the exercise of the nutritive functions.
The weight of the body is reduced to the lowest possible limit consistent with its general size and the exercise of its functions. The limbs, for instance, take no part in the nutritive function, and are reduced in size to the weight of bone requisite for support and of muscle necessary to move the body. The head is just large enough to hold the brain, the organs of special sense, and the mouth with its
