84 THE WHENCE AND THE WHITHER OF MAN 



vertebrae persisted through the oldest amphibia, rep- 

 tiles, and birds. But finally a firm backbone and skull 

 were attained. 



2. The appendages. Of these we can say but little. 

 The fish has oar-like fins, attached to the body by a 

 joint, but themselves unjointed. By the amphibia 

 legs, with the same regions as our own and with five 

 toes, have already appeared. The development of the 

 leg out of the fin is one of the most difficult and 

 least understood problems of vertebrate compara- 

 tive anatomy. The legs are at first weak and scarcely 

 capable of supporting the body. Only gradually do 

 they strengthen into the fore- and hind-legs of mam- 

 mals, or into the legs and wings of birds and old flying 

 reptiles. 



3. Changes in the circulatory and respiratory sys- 

 tems. The fish lives altogether in the water and 

 breathes by gills, but the dipnoi among fishes breathes 

 by lungs as well as gills. As long as respiration takes 

 place by gills alone, the circulation is simple ; the 

 blood flows from the heart to the gills, and thence 

 directly all over the body; the oxygenated blood from 

 the gills does not return directly to the heart. But the 

 blood from the lungs does return to the heart ; and 

 there at first mixes in the ventricle with the impure 

 blood which has returned from the rest of the body. 

 Gradually a partition arises in the ventricle, dividing 

 it into a right and left half. Thus the two circulations 

 of the venous blood to the lungs, and of the oxygen- 

 ated blood over the body, are more and more separated 

 until, in higher reptiles, they become entirely distinct. 



As the animal came on land and breathed the air, 

 more completely oxygenated blood was carried to the 



