Nox. 2:4] THE HARD PARTS OF THE MAMMALIA. 149 
articulations which are flexed, extended, and rotated. Such are 
especially subject to the laws of motion, and it is in them that 
the effects of the latter are distinctly seen. As an example, the 
ankle joint is in the primitive Condylarthra, a flat joint, or not 
tongued or grooved. In most of the Carnivora, in a few Roden- 
tia, and in all Diplarthra, it is deeply tongued and grooved, form- 
ing a more perfect and stronger joint than in the other orders, 
where the surfaces of the tibia and astragalus are flat, as in the 
Condylarthra. 
I. 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, Rodentia, Insectivora, Edentata, have 
short legs, with a few cases of extreme specialization as ex- 
ceptions, such as Kangaroos, Rabbits, and Jerboas (hind legs 
only), the Dolichotis patachonica, the Rhynchocyonidze 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 respective orders. Thus in Diplarthra, the 
deer, antelope, and horse are distinguished for length of limb ; 
in the Proboscidia, the elephant; in the Carnivora, the large 
cats and hyenas; 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 time. The mechanical character of that use 
has not been identical. It is of two principal kinds, viz. : 
