154 COPE. [Vot. III. 
and calcaneum are elongate. In the bats they are still more so, 
and do not possess any distinct trochlea. The greater modifica- 
tion of the tarsus in the bats is due to the fact that the order 
Chiroptera dates from the lower Eocene epoch, while the arbo- 
real sloths originated in the Pliocene; also, perhaps, because 
they are suspended from the hind legs only, while the sloths 
hang from all four. In the latter the carpus does not share 
considerably in the elongation of the tarsus, but the phalan- 
ges are greatly elongate. In the Chiroptera the heel of the 
calcaneum is the line of attachment of a dermal membrane 
through which it has experienced a continual lateral strain. 
Concomitantly, it has grown to an enormous length. It is 
not so easy to understand the mechanical law which ex- 
plains the growth of a part through lateral strain as through 
longitudinal strain, It is, however, not difficult to understand 
why this very elongate heel should become segmented off from 
the body of the calcaneum by the lateral strain of the caudal 
membrane, as it has. 
y. Modifications through Other Use and Disuse. 
That the loss of articular condyles follows from a disuse of 
the articulation may be readily shown. That a segment of a 
limb may be shortened by a transfer of its function to some 
other segment may be also demonstrated. That muscular in- 
sertions have been enlarged or diminished directly as the use 
of the muscles inserted in them, is apparent from the facts. 
The enormously developed muscular insertions of the hume- 
rus in mammals which dig are well known. Illustrations of this 
are seen in the existing monotremes, Platypus and Tachyglossus, 
the armadillos, and still more strikingly in the moles. Large 
development of unequal phalanges is characteristic of all these 
animals, but it reaches excessive proportions in the Cape ant- 
eater Orycteropus, and in the giant armadillo. Huge develop- 
ment of the muscular insertions of the humerus and of the 
scapula are especially noticeable in animals which pull the vege- 
tation on which they feed to them from elevated positions or 
from below the earth, as in the extinct edentates of the fam- 
ily Megatheriide. As the sloths grew smaller in dimensions, 
the trees refused to yield to the diminished prehensile power of 
