KINETOGENESIS. 305 



are, however, probably due to the same process, viz., 

 impact and strain. 



a. The Proportions of the Limbs and of Their Segments. 



The length of the legs of terrestrial Mammalia has 

 increased with the passage of time. The inferior types 

 of Mammalia now existing, as Marsupialia, Glires, 

 Insectivora, Edentata, have short legs, with a few ' 

 cases of extreme specialization as exceptions, such as 

 kangaroos, rabbits, and jerboas (hind legs only), the 

 Dolichotis patachonica, the Rhynchocyonidse, and the 

 sloths. In the orders which stand at the summit of 

 the series, as the Diplarthra, Proboscidia, Carnivora, 

 and Anthropomorpha, the legs are much increased in 

 length, and this is especially marked in certain forms 

 which stand in all respects at the summit of their re- 

 spective orders. Thus in Diplarthra, the deer, ante- 

 lope, and horse are distinguished for length of limb ; 

 in the Proboscidia, the elephant ; in the Carnivora, 

 the large cats and hysenas ; in the Anthropomorpha, 

 the fore limbs are long in all, the hind ones especially 

 so in man. 



The cause of this elongation is apparently use. It 

 is the hind legs that are elongated in a straight line in 

 animals that walk on them, as man ; and both, in those 

 that walk on both, as the elephant. In animals that 

 leap with the hind legs these are still more elongated, 

 and are folded when at rest, and rapidly extended 

 when in motion. In animals that climb with the fore 

 legs, these are elongated, as in the Anthropomorpha, 

 except man. In those that climb with all fours, all 

 are elongate, as in the sloths. It must be remembered 

 that these elongations are the sum of increments added 

 one to the other through long ages of use in geologic 



