VARIATION IN TIME 153 



mentioned series links together species, such as 

 the Rhinoceros Schleiermacheri and the Rhinoceros 

 pachygnathus, which have nothing in common 

 and correspond to the parallel evolution of two 

 distinct branches. It may, therefore, be said that 

 Gaudry has studied, not the evolution of the 

 rhinoceros group, but simply the gradual thickening 

 of the bones intended to support the horns in a 

 series of genera in no way related. Almost similar 

 functional series might be described in all the other 

 groups of horned vertebrates. 



The functional organization of a paw destined for 

 burrowing in the earth gives rise to almost identical 

 structures, whatever the group to which the burrow- 

 ing animal may belong. The ungual phalange be- 

 comes very large, and is provided with a groove 

 to receive the insertion of the horny claw. But 

 this burrowing arrangement is awkward for pro- 

 gression, so that special dispositions of the articu- 

 lations of the joints with the metacarpus allow 

 the animal to raise the digits and to walk on the 

 palm of the hand. This structure may be ob- 

 served more or less developed in several genera 

 of existing animals, the Tatus, Pangolins, etc., 

 forming part of the order a not very natural one, 

 by the by of the Edentata. Misled by his con- 

 stant habit of comparing functional adaptations, 

 Gaudry could not fail to see a transition between 

 the Edentata and the Ungulata, in that curious 

 family of Tertiary Mammals, the Schizotherium, the 

 Macrotherium, and the Chalicoiherium, in which the 

 front paw presents in the highest degree the burrow- 

 ing structure above described. But, in reality, 



