No. 2.] THE HARD PARTS OF THE MAMMALIA. 157 
of the foot against the water. This is followed by the suc- 
cessive shortening of the femur, which from being a lever has 
become a fulcrum for the tibia and hind foot. In this position, 
where it is comparatively fixed, it undergoes successive abbre- 
viation and thickening, until it assumes the form characteristic 
of the seals (Figs. 6, 7). 
Use and disuse play an important part in the determination 
of the forms and proportions of the phalanges. In oar-like 
limbs adapted to swimming, like the other segments, they lose 
their condyles and become immovable on 
each other, as in the Cetacea and Sirenia. 
In the seals the flexibility of the digits is 
greater, as their line of aquatic ancestors 
is shorter. They add an especial pecul- 
iarity in the superior size of the internal 
digit of the fore foot, and of the internal 
Figure 7.— Anterior and posterior legs of the seal A/onachus albiventer ; from 
Cuvier. 
and external digits of the hind foot. This must be ascribed 
to the strains incident on use, and the consequent increased 
nutrition of the parts. Seals constantly rest on shore and on ice- 
floes, in a manner unknown to both Sirenia and Cetacea. The in- 
