158 STUDIES IN PHYSIOLOGY 



a single ventricle. In frogs and toads (the amphibia) we 

 find the beginnings of a right and left heart. But while 

 the separation of the auricles is complete, the two kinds of 

 blood are mixed in the single ventricle. The two sides of 

 the heart are more completely separated in the reptiles, and 

 when we come to the birds and mammals we find that there 

 is no means of direct communication between the right and 

 left hearts. Hence, in the highest groups, there is a dis- 

 tinct pulmonary and systemic circulation. 



In our comparative study of digestion, we noted an increas- 

 ing complexity of structure from the earthworm, through 

 the frog and bird, to the mammal. The same fact is evi- 

 dent in considering the various circulatory systems. The 

 higher animals have more complete machinery for doing 

 the work of the body, and for this reason they can carry on 

 the processes of life more perfectly. 



