90 The American Naturalist. [January, 



been direct. It is only necessary to call attention to the leading facts, 

 now well known, thanks largely to the investigations of Americans, of 

 the evolution of the vertebrate skeleton. 



He commenced with the highest class, the mammalia, where the 

 evidence is very complete. Such is the simple fact if mum rical digital 

 reduction from five in the lower ungulates through the numbers four 

 and three to one, as in the horse ; or to four and two, as in the ox and 

 deer. Then carrying the line of variation towards the central parts 

 of the skeleton, Professor Cope described the articulation of the limbs. 

 The development of keels on the metacarpals was mentioned ; then 

 the development of facets on the radius at the wrist. Next, the devel- 

 opment of the tongue and groove articulation between the radius and 

 ulna proximally, and next the same in the humerus and radius at the 

 elbow. The successive reduction of the ulna was mentioned. The 

 hind limb was next considered, and the progressive process of devel- 

 opment was described. The intervertebral articulations were then dis- 

 cussed, and their successive modifications in the artiodactyla described. 



These characters all indicate a direct variation of individuals in 

 the direction of perfect mechanical contrivances in the skeleton. 



Professor Cope then referred to the presence of the same phenome- 

 non in the dentition of mammalia. He dwelt especially on the evolu- 

 tion of the sectorial teeth of carnivorous forms, from the tritubercular 

 upper molar and tuberculosectorial inferior molar. 



The successive variations seen in the reptilian skeleton were then 

 referred to. The development of fins from ambulatory limbs in the 

 Ichthyosauria ; next of upright walking types, like the birds, in the 

 Dinosauria, with the pelvic bones thrown back to sustain the weight 

 of the viscera in the same position. Next the evolution of the modern 

 types of lizards and snakes by the development in the length of the 

 suspensoria of the lower jaw to enlarge the gape for swallowing ; and 

 second, in the loss of the capitular articulations of the ribs, due to the 

 support of the weight on the ground, just as occurs in the Plesiosau- 

 rian reptiles and in the whales, where the body is supported by the 



Professor Cope stated that his conclusion from these and many other 

 similar facts was that the origin of such variations had not been pro- 

 miscuous or fortuitous, but direct, and in consequence of the operation 

 of a definite cause. That cause he believed to be growth energy (of 

 which we know little or nothing), directed by the mechanical rela- 

 tions between the animal and its e 



