to counteract a tendency to unsymmetrical growth,
where one side of the body is used more than the other. But the
undoubted hurtfulness of want of symmetry in many important actions or
functions would rapidly eliminate any such tendency. When, however, it
has become useful, as in the case of the single enlarged claw of many
Crustacea, it has been preserved by natural selection.

_Origin of the Feet of the Ungulates._

Perhaps the most original and suggestive of Mr. Cope's applications of
the theory of use and effort in modifying structure are, his chapters
"On the Origin of the Foot-Structure of the Ungulates;" and that "On the
Effect of Impacts and Strains on the Feet of Mammalia;" and they will
serve also to show the comparative merits of this theory and that of
natural selection in explaining a difficult case of modification,
especially as it is an explanation claimed as new and original when
first enunciated in 1881. Let us, then, see how he deals with the
problem.

The remarkable progressive change of a four or five-toed ancestor into
the one-toed horse, and the equally remarkable division of the whole
group of ungulate animals into the odd-toed and even-toed divisions, Mr.
Cope attempts to explain by the effects of impact and use among animals
which frequented hard or swampy ground respectively. On hard ground, it
is urged, the long middle toe would be most used and subjected to the
greatest strains, and would therefore acquire both strength and
development. It would then be still more exclusively used, and the extra
nourishment required by it would be drawn from the adjacent less-used
toes, which would accordingly diminish in size, till, after a long
series of changes, the records of which are so well preserved in the
American tertiary rocks, the true one-toed horse was developed. In soft
or swampy ground, on the other hand, the tendency would be to spread out
the foot so that there were two toes on each side. The two middle toes
would thus be most used and most subject to strains, and would,
therefore, increase at the expense of the lateral toes. There would be,
no doubt, an advantage in these two functional toes being of equal size,
so as to prevent twisting of the foot while walking; and variations
tending to bring this about would be advantageous, and would therefore
be preserved. Thus, by a parallel series of changes in another
direction, adapted to a distinct set of conditions, we should arrive at
the