VI.] THE COMMON FROG. 65 



the continuity of the dorsal plates with the subjacent 

 joints of the backbone — should have arisen twice in 

 nature spontaneously. Here we seem to have a 

 remarkable example of the independent origin of 

 closely similar structures ; and if so, what caution is 

 not necessary before concluding that any given 

 similarity of structure are undoubted marks of 

 genetic affinity ! 



The skin of the frog is also interesting from a 

 physiological point of view. Our own skin is by no 

 means popularly credited with the great importance 

 really due to it. " Only the skin ! " is an exclatnation 

 not unfrequently heard, and wonder is very often felt 

 when death supervenes after a burn which has injured 

 but a comparatively small surface of the body. Yet 

 our skin is really one of our most important organs, 

 and is able to supplement, and to a very slight extent 

 even to replace, the respective actions of the kidneys, 

 the liver, and the lungs.^ 



In the frog we have this cutaneous activity de- 

 veloped in a much higher degree. Not only does its 

 perspiratory action take place to such an extreme 

 degree that a frog tied where it cannot escape the 

 rays of a summer's sun speedily dies — nay, more, is 

 soon perfectly dried up — but its respiratory action is 

 both constant and important. This has been experi- 

 mentally demonstrated by the detection of the car- 

 bonic acid given out in water by a frog over the head 

 of which a bladder had been so tightly tied as to 

 prevent the possibility of the escape of any exhalation 

 from the lungs. The fact of cutaneous respiration 



* See "Elementary Physiology," Lesson V., § 19. 



F 



