192 COPE. [Vot. III. 
of the tibia to embrace the side walls of the astragalus. Now, 
this is exactly what would happen if two pieces of similar dead 
material, similarly placed, should be subjected to a continual 
pounding in the direction of their length for a long period of 
time. And we cannot ascribe any other immediate origin to it 
in the living material ; but the probability of such origin is mére 
probable in such substance, because of the perpetual waste and 
repair which are going on, as illustrated by the power which 
we see in growth, in repairing damages, and in providing for 
new conditions in cases of accident. The inclusion of the 
astragalus in the tibia does not occur in the Reptiles, but appears 
first in the Mammalia, which descended from them. 
The same active cause that produced the two grooves of the 
lower end of the leg produced the groove of the middle of the 
upper end of the astragalus. Here we have the yielding lower 
end of the tibia resting on the equally spongy material of the 
middle of the astragalus. There is here no question of the hard 
material cutting into soft, but simply the result of continuous 
concussion. The consequence of concussion would be to cause 
the yielding faces of the bones to bend downward in the direc- 
tion of gravity. If they were flat at first they would begin to 
hollow downward, and a tongue above and a groove below would 
be the result. And that is exactly what has happened. With- 
out exception, every line of Mammalia commenced with types 
with an astragalus which is flat in the traverse direction, or 
without median groove. From early tertiary times to the 
present day, we can trace the gradual development of this 
groove in all the lines which have acquired it. The upper sur- 
face becomes first a little concave; the concavity gradually 
becomes deeper, and finally forms a well-marked groove. 
The history of the wrist-joint is similar. The surface of the 
fore-arm bones which joins the fore foot is in the early tertiary 
Mammalia uniformly concave. In the ruminating mammals it 
is divided into three fossze, which are separated by sharp keels. 
These fossze correspond with the three bones which form the 
first row of the carpus or palm. The keels correspond to the 
sutures between them. The process has been evidently similar 
to that which has been described above as producing the side 
grooves in the end of the tibia. The dense walls of the sides 
of the three bones impinging endwise on the broad yielding sur- 
