THE AMPHIBIA 95 



had to be remodeled for vision in air, with the conse- 

 quence that the brain became increasingly concerned 

 with the sense of sight. For the first time the animal 

 began to breathe through the nose, thus bringing the 

 *nose-brain' into a role of greater importance. (It was 

 from this nose-brain' that the cerebral hemispheres of 

 the mammals were to be evolved at a later time.) And 

 so it was with almost every organ— some greater or lesser 

 transformation contributed to this, one of the most revo- 

 lutionary events in vertebrate history. 



It is something of a paradox, in view of the many other 

 structural changes in this class, that the evolution of the 

 Amphibia saw no marked change in the structure of the 

 kidney: in all recent forms the glomeruh remain large 

 and well vascularized and the tubule consists, as in the 

 fresh- water fishes, of the typical proximal and distal seg- 

 ments. There can be no doubt that the Carboniferous 

 Amphibia, like recent forms, were incapable of elaborat- 

 ing a urine osmotically more concentrated than the 

 blood, and it was this circumstance, as much as any- 

 thing, that Hmited their habitat. Like Hving salamanders 

 and frogs, they had to spend most of their fives in the 

 water or at best, like the toads, not too far away on 

 moist ground or in a moist nest of moss or leaves. Never- 

 theless, they added two patents which served the ends 

 of water balance: first, a mechanism for reducing the 

 rate of glomerular filtration (and hence the excretion of 

 water) when they were out of water; and second, a 

 mechanism for varying the permeability of the skin to 

 water. Both of these mechanisms are governed by the 

 pituitary gland. 



Occasion has not hitherto arisen to speak of any of 

 the glands of internal secretion (the endocrine glands) 

 which are so important in the physiological activities of 

 the higher animals, not because such glands are lacking 

 in the fishes but because so little is known about them. 

 However, one of these glands, the pituitary, which is 



