running and sprinting--for speed. Now, we do find among the various
races of mankind that some have been given long heels, such as the
dark-skinned natives of Africa and of Australia, while other races have
been given relatively short, stumpy heels, of which sort the natives of
Europe and of China may be cited as examples. With long heels less
powerful muscular engines are required, and hence in dark races the calf
of the leg is but ill developed, because the muscles which move the heel
are small. We must admit, however, that the gait of dark-skinned races
is usually easy and graceful. We Europeans, on the other hand, having
short heels, need more powerful muscles to move them, and hence our
calves are usually well developed, but our gait is apt to be jerky.

[Illustration: Fig. 6.--The bones forming the arch of the foot, seen
from the inner side.]

If we had the power to make our heels longer or shorter at will, we
should be able, as is the case in a motor cycle, to alter our
"speed-gear" according to the needs of the road. With a steep hill in
front of us, we should adopt a long, slow, powerful heel; while going
down an incline a short one would best suit our needs. With its
four-change speed-gear a motor cycle seems better adapted for easy and
economical travelling than the human machine. If, however, the human
machine has no change of gear, it has one very marvellous
mechanism--which we may call a _compensatory_ mechanism, for want of a
short, easy name. The more we walk, the more we go hill-climbing, the
more powerful do the muscular engines of the heel become. It is quite
different with the engine of a motor cycle; the more it is used, the
more does it become worn out. It is because a muscular engine is living
that it can respond to work by growing stronger and quicker.