Chap. II.] EXTERNAL CHARACTERS. 19 



we have seen to be primitive in the newt. In its closed position 

 it is bent upon itself Z-fashion, the 

 brachium and the manus being directed 

 backwards, and the antibrachium for- 

 wards. (Fig. 6.) In the hind limb, 

 the folding forwards which we noticed 

 in the rabbit has taken place. But the 

 undivided portion of the pes is so 

 thoroughly raised off the ground, that 

 it appears to belong rather to the leg 

 than to the foot, and, indeed, as we 



shall see hereafter, belongs partly to the one and partly to the 

 other. 



General Conclusions. Thus in these vertebrate types there 

 is much fundamental resemblance overlain and partially masked 

 by many well-marked differences. In the possession of a skull 

 and vertebrated back-bone ; in the mode of attachment of the 

 limbs, and their general structure in frog, pigeon, and rabbit ; 

 in the position of mouth, nares, eyes, and ears, there is resem- 

 blance. In the possession of a common cloaca, the frog and the 

 pigeon differ from the rabbit, as this in turn does from the fish, 

 in having a common urino-genital aperture. In the absence of 

 exoskeleton, the frog differs from the other types. The scales 

 . of the fish are dermal "structures formed in the deeper layer of 

 the skin. Feathers and hairs are epidermal structures formed in 

 the superficial layer of the skin. In their modes of life each 

 differs from the other. The cod-fish is aquatic and marine ; the 

 frog amphibious near fresh water ; the rabbit is terrestrial ; the 

 pigeon fitted for aerial life. Hence the special modification of 

 the limbs and the form of the body. The cod breathes oxygen 

 dissolved in sea-water, the basic salts of which absorb the car- 

 bonic acid expired : the tadpole breathes the oxygen dissolved 

 in fresh water : the frog, pigeon, and rabbit breathe the oxygen 

 of the air, but the frog requires less than one-tenth of that 

 required by the pigeon. In their food they differ widely. The 

 cod feeds on shell-fish and other marine animals ; the tadpole 



