No. 2. } THE HARD PARTS OF THE MAMMALIA. 155 
the fore legs. The result has been that the sloths ascended the 
trees, and their limbs assumed more and more the character of 
mere suspensors of the weight of the body. Thus the muscular 
insertions of the humerus diminished, and their humeri are to- 
day furnished with weak ones only, the greater and lesser tuber- 
osities of the head having nearly disappeared, so that the head 
of the humerus resembles that of other arboreal mammals, as 
the Quadrumana and Simiide. 
The limbs have undergone great modifications of form in 
their gradual adaptation to aquatic 
habits. The stages of this process 
are to be observed first in the sea- 
otter (Enhydra), then in the seals, 
then in the sirenians, and lastly 
in the Cetacea. This succession 
is not given here as a phylogeny, 
for paleontology does not warrant 
any such history. 
The use of a limb as an oar for 
propulsion in the water requires 
that it shall be, so far as the blade 
is, Concermed, inflexible, “Such a 
structure has existed in all thor- / 
oughly aquatic vertebrata. This 
implies the immobility of the ar- 
ticulations, which is due to the 
loss of their condylar surfaces. 
This may be traced to disuse of 
such articulations. This disuse 
would be at first voluntary, the 
limb being held stiffly while used , cA te Bie eae 
as an oar in thes acieos swim- hei ME PIORU Ce et 
ming. Loss of power of extension 
and flexion is well known to result from disuse. It is well known 
that the flexors and extensors of the manus have become atro- 
phied in the Cetacea. Not so, however, with the flexors and 
extensors of the humerus, which become those of the entire 
limb. In the whales the first segment of the fore limb is en- 
closed within the integument of the body, so that its motion 
being much restricted, the insertional crests are reduced in size. 
