OF EVOLUTION. 29 



heart, the presence of wings, and certain struc- 

 tures connected with the hinder extremities. But 

 the first distinction is immediately disposed of by 

 the case of the crocodiles and alligators, which, 

 alone among reptiles, have the four recognized 

 chambers of the heart belonging to the highest 

 animals. The matter* of wings is also disposed of 

 by those remarkable reptilian creatures belonging 

 to the same epoch of geological time as the 

 earliest bird, the pterodactyls, which in many 

 other characters such as the light structure and 

 manner of support of the head, the presence of a 

 well-developed keel or carina to the breast-bone, 

 etc. also approximate the birds. Furthermore, 

 we are well aware that in the large group of stru- 

 thious birds the ostriches, cassowaries, and apter- 

 yxes the wings are so little developed as to be 

 all but functionless. We are hence driven to the 

 remaining characters derived from the structure 

 of the hind-quarters and their appendages. 



The most careless observer is aware that a bird 

 can at almost all times be distinguished from a 



