KINETOGENESIS. 361 



sued in the Marsupialia, Glires (Lepus, Dolichotis, 

 Dipus), Carnivora, Ungulata, Quadrumana. 



Eighth, reduction of digits has occurred under sim- 

 ilar conditions in Marsupialia, Glires, Insectivora, 

 Carnivora, Ungulata. 



Ninth, the atrophy of the ulna and fibula occur in 

 the distinct lines of the Perissodactyla and Artiodac- 

 tyla, and the atrophy of the fibula in the leporid 

 Glires ; all in limbs which function in the most rapid 

 progression. 



Further confirmation of the law of kinetogenesis is 

 to be found in those cases where different structures 

 appear in different parts of the skeleton of the same 

 individual animal, in direct correspondence with the 

 different mechanical conditions to which these parts 

 have been subjected. Examples : the diverse modifi- 

 cations of the articulations of the limbs in consequence 

 of the uses to which they have been put, in mammals 

 which excavate the earth with one pair of limbs only; 

 as in the anterior limbs of the fossorial Edentata, In- 

 sectivora, and Glires. The reduction of the number 

 of the digits in the posterior limb only when this is 

 extensively used for rapid progression, as in leaping: 

 this is seen in the kangaroo and jerboas, in the orders 

 Marsupialia and Glires. 



The development of a dental structure of premolars 

 identical with that of the molars, from a different struc- 

 tural origin, in the Perissodactyla. 



From the preceding facts I have inferred that in 

 biologic evolution, as in ordinary mechanics, identical 

 causes produce identical results. 



