288 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



now discovered the outlines of the phylogeny of many 

 mammalian types, and many detailed histories of spe- 

 cial lines of descent are known. Our knowledge is 

 most complete in the unguiculate and ungulate pla- 



centals, while it is least 

 as regards the Mutilata, 

 and the implacentals. We 

 have excellent series of 

 skeletal parts, and I have 

 given the successional 

 modifications of some of 

 them on page 139. 



In the first place, I 

 will select an illustration 

 of the effects of use on 

 the articulations of the 

 limbs and feet of the 

 Mammalia. I take first 

 the ankle and wrist-joints. 

 In the ruminating animals 

 (ox, deer, camel, etc.) and 

 in the horse, among other 

 living species, the ankle- 

 joint is a very strong one, 



j j -, r 



^nd yet admits of an CX- 



tensive bending of the foot 

 ing pentadactyle plantigrade foot with- ., i j . tre U] e 



out groove of astragalus, as in the C 

 probable ancestor of the Diplarthra. tOngUC-and-grOOVC joint ; 



that is, two keels of the 



first bone of the foot, the astragalus, fit into two grooves 

 of the lower bone of the leg, the tibia, while between 

 these grooves a keel of the tibia descends to fill a cor- 

 responding groove of the astragalus. Such a joint as 

 this can be broken by force, but it cannot be dislocated. 



Fig. 68. Periptychus rhabdodon 

 Cope, a condylarthrous genus of the 

 Puerco epoch of New Mexico; poste- 

 rior foot, one-half natural size, show- 



