270 ANNALS NEW YORK ACADEMY OF SCIENCES 



Evolutionary Stages of Quadrupedal Locomotion 



In the Stegocephalian stage of quadrupedal locomotion, the short limbs 

 were held widely outward from the body, the humerus and femur were 

 very short and the feet were spreading and flat. Crawling was effected 

 in part by a sharp downward pull of a proximal segment (humerus or 

 femur), thus tilting the body upward on the same side and throwing the 

 weight on the opposite foot. The long axis of the body was meanwhile 

 thrown into alternate lateral curves, the advancing fore limb being on a 

 convexity, the advancing hind limb on a concavity. 



In the late reptilian or early mammalian stage, the feet were brought 

 around partly under the body, the elbow and knee began to be drawn in, 

 the scapula was rotated backward as the coracoid lost its connection with 

 the sternum, and the body became well raised off the ground. According 

 to a hypothesis advanced elsewhere by the writer, 15 this process was asso- 

 ciated with the acquisition of climbing, or semi-arborea'l, habits, struc- 

 tural vestiges of which remain in the partly divergent first digit and many 

 other characters of early Eocene mammals. 16 



The Lower Eocene ancestors of the various orders of ungulates had 

 probably all long since passed through these earlier stages of quadrupedal 

 terrestrial locomotion, and at that time many of them had perhaps become 

 more or less digitigrade. The primitive "Protungulates," Meniscotherium, 

 Periptychus, Pantolambda, may give us some idea of what the several 

 ancestors of the sub ungulate series (Hyracoidea, Embrithopoda, Pro- 

 boscidea, Amblypoda) may have been like. They also preserve apparent 

 traces of arboreal ancestry in the relatively short, spreading hands and 

 feet, long limb bones, the humerus with large entocondyle and entepicon- 

 dylar foramen, the undiminished power of pronation and supination of 

 the forearm and many other characters. The Basal Eocene Euproto- 

 gonia, the ancestor of the Condylarth Phenacodus, with slender subun- 

 guligrade feet, represents a more advanced stage of ' evolution, in the 

 direction of the Perissodactyls. 



Factors of long-distance Travelling Power in Ungulates 



The primitive ungulates of the Lower Eocene were doubtless sur- 

 rounded by environmental conditions which set the premium of survival 

 upon improvements in long-distance travelling power and in speed. 

 These improvements have been attained in various ways and in the most 



15 "The Orders of Mammals," Bull. Amer. Mus. Nat. Hist, Vol. 27, p. 226, 1910. 



16 Matthew : Arboreal Ancestry of the Mammalia, Amer. Nat., Vol. 38, 1904, pp. 

 811-818. 



