FINS AND FINGERS. 125 



througli the water, they had now also to afford support to 

 the animal while creeping upon land. This effected a 

 modification both of the skeleton and of the muscles of 

 the limbs. The number of fin rays was gradually lessened, 

 and was finally reduced to five. These five remaining 

 rays now, however, developed more vigorously. The soft 

 cartilaginous rays became hard bones. The rest of the 

 skeleton also became considerably more firm. The move- 

 ments of the body became not only more vigorous, but 

 also more varied. The separate portions of the skeleton 

 system, and consequently those of the muscular system also, 

 became more and more differentiated. Owing to the intimate 

 correlation of the muscular to the nervous system, the latter 

 also naturally made marked progress in point of function 

 and structure. We therefore find that the brain is very 

 much more developed in the higher Amphibia than in 

 Fishes, in Mud-fishes, and in the lower Amphibia. 



The organs which are most modified in consequence of 

 an amphibious mode of life are, as we have already seen in 

 the Double-breathers (Dipneusta), those of respiration and 

 of the circulation of the blood. The first advance in 

 organization necessitated by the transition from aquatic to 

 terrestrial habits of life was, of course, the formation of an 

 air-breathing organ, a lung. This developed directly from 

 the swimming-bladder which these animals had inherited 

 from the Fishes. At first the function of this organ would 

 be quite subordinate to the more ancient organ, used for the 

 respiration of water, the gills. Hence we find that the 

 lowest Amphibia, the Gilled Amphibia, like the Dij)neusta, 

 spend the greater part of their lives in the water, and that 

 accordingly they breathe water through gills. It is only 



