OF EVOLUTION. 31 



reptile by its mode of progression its elevation 

 on the hinder extremities alone. But this mode 

 of progression does not differ" more from that of a 

 reptile than does the method of a snake from that 

 of a turtle, yet both are reptiles. It is in the relative 

 disposition of the parts that we find the important 

 difference. In all birds the pelvic girdle, which 

 consists of the three bones recognized in man as 

 the pubis, ischium, and ilium, has the pubis 

 directed in a direction more or less parallel with 

 that of the ischium ; in other words, backward. 

 In all reptiles, on the other hand, this bone is 

 directed forward. Again, in all, or nearly all 

 birds, there is a prominence, known as the cnemial 

 crest, developed on the upper part of the tibia, 

 for the attachment of the powerful muscles of the 

 leg. This is wanting in reptiles ; and further, 

 there are certain peculiarities connected with the 

 articulation of the foot to the leg in birds which 

 almost immediately serve to distinguish these 

 parts from the similar parts of reptiles. Have 

 we any reptilian forms which at all meet the 



