128 TIIK MAMMALIA. 



fourth toe ; hence these two toes are more active 

 than the side toes, and have an almost equal 

 amount to do in bearing the weight. They ac- 

 quire strength l>y this function, while the less active 

 toes gradually become useless and finally disap- 

 pear. \Y< have already referred to the importance 

 of the specialisation of the teeth, and to the sim- 

 plification of the limbs produced by the reduction 

 and disappearance of the toes, as an advance for 

 certain functions. And it is self-evident that the 

 disappearance of the toes must, in a lesser or 

 greater degree, affect the middle part of the hand 

 and foot, the roots of the hand and foot, the bones 

 of the lower arm and leg, and also the muscles and 

 other soft parts of the limbs. 



Kowalewsky has pointed out an exceedingly in- 

 teresting circumstance connected with the bones of 

 tin- hand and foot, and distinguishes tHatlujrtiri' and 

 <i<lnptir,> forms among the Hoofed animals, \\here 

 the toes are either partly rudimentary or have 

 completely disappeared. This explanation of Kow- 

 alewsky's throws some light upon the question 

 why so many lateral branches of the Mammalia 

 have died out, whereas previously we had to be 

 satisfied with the general supposition that the-e 

 were the very branches that had not survived in 



