256 THE THEORIES OF EVOLUTION 



cussion would be to cause the yielding faces of the 

 bones to bend downward in the direction of gravity, 

 or to remain in their primitive position while the edges 

 of the astragalus were pushed into the tibia. If they 

 were flat at first they would begin to hollow down- 

 ward, and a tongue above and groove below would be 

 the result. And this is exactly what has happened. 

 This inclusion of the astragalus in the tibia does not 

 occur in the reptiles, but appears first in the Mam- 

 malia, which descended from them." . . . 



"I have shown that without exception, every line of 

 Mammalia commenced with types with an astragalus 

 which is flat in the transverse direction, or without me- 

 dian groove (Periptychus rhabdodon of New Mex- 

 ico)." . . . "From early Tertiary times to the 

 present day, we can trace the gradual development of 

 this groove in all the lines which have acquired it. 

 The upper surface became first a little concave 

 (Pcebrotherium labiatum of Colorado), the con- 

 cavity gradually became deeper, and finally formed 

 a well-marked groove (Prothippus sejunctus of 

 Colorado). The history of the wrist- joint is sim- 

 ilar." . . . 



"There is another striking instance of the same kind 

 in the feet of Mammalia ; that is, in the development 

 of the keels and grooves which appear at the articula- 

 tion of the first set of bones of the toes (metapodials) 

 with the bones of the second set (phalanges). A 



