292 rRIMARY FACTORS OF ORGANIC EVOLU7FON. 



gravity, remains to fill the 

 grooves, and to form the 

 keels which we observe. (See 

 Fig. 72.) 



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 ap- 

 pear at the articulation of the 

 first set of bones of the toes 

 (metapodials) with the bones 

 of the second set (phalanges). 

 These keels first appear on 

 the posterior side of the end 

 of the first set of bones, pro- 

 jecting from between two 

 fiexor tendons. These ten- 

 . dons, in many mammals, con- 

 tain two small bones, one on 

 each side, each of which acts 

 like the knee-pan, and resem- 

 bles it in miniature, which are 

 called sesamoid bones. These 

 tendons and bones exercise 

 a constant pressure on each 

 side of the middle line, when 

 the animal is running or 

 walking, and this pressure, 

 together with the concussion 

 with the ground, appears to 

 have permitted the protru- 

 sion of the middle line in the 

 form of a keel, while the 



