152 COPE. [Vot. III. 
itigrade animals it is the feet which receive the impact of the 
repeated blows on the earth while in progression, while support- 
ing the weight of the body at every stage of the process. In 
plantigrade animals it is the soles of the feet and the bones 
of the leg in line with them, which receive the impact, while 
the feet beyond this point receive none, and do not support 
the body except very partially at the moment of leaving the 
earth. In the case of some of 
the Quadrumana, the prehensile 
use of the feet renders any con- 
siderable impact impossible, so 
the feet and hind leg generally 
have remained short. But in the 
case of the Tarsius, the habit 
of leaping has been added to the 
functions of arboreal progres- 
sion, and the result has been 
a remarkable elongation of the 
posterior foot. But it has not 
been the metapodials and digits 
which have experienced this 
Figure 3. — Tarsius spectrum, poste- elongation, since they have been 
rior extremity, external side: from De always employed in prehension. 
Blainville. It has been the tarsus proper, 
the astragalus, and calcaneum, whose distal extremity is the 
very point which in a plantigrade animal must receive the impact 
of a leap under such circumstances. 
In all of the above cases the impact is more or less directly 
longitudinal, or in the direction of the length of the bone, at 
some stage of the act of progression. 
B. Increase of Length by Stretching. 
Examples of this kind of stimulus to the elongation of bones 
of the limbs are to be seen in those animals which are, for a 
greater or less part of the time, suspended from the limbs of 
trees. Such stretching must be, it is evident, in excess of mus- 
cular resistance in the opposite direction, or an opposite effect 
results. Thus the sloth hangs suspended from the limbs of trees, 
exerting apparently no muscular effort in the act, maintaining 
