esirable character might run for a somewhat longer period, but
even this would tend toward disappearance and elimination by the stern,
selective influence of environment.

Naturally, this great conservative tendency of nature has, like all
other influences, "the defects of its virtues," as the French say. It
has no gifts of prophecy, and in the process of handing down to
successive generations those mechanisms and powers which have been
found useful in the long, stern struggle of the past, it will also hand
down some which, by reason of changes in the environment, are not only
no longer useful, but even injurious. As the new light of biology has
been turned on the human body and its diseases, it has revealed so many
of these "left-overs," or remnants in the body-machine--some of most
dramatic interest--that they at first sight have done much to justify
the popular belief in the malignant tendencies of heredity.

Yet, broadly considered, the overwhelming majority of them should really
be regarded as honorable scars, memorials of ancient victories,
monuments to difficulties overcome, significant and encouraging
indications of what our body-machine is still capable of accomplishing
in the way of further adjustment to conditions in the future. The really
surprising thing is not their number, but the infrequency with which
they give rise to serious trouble.

The human automobile is not only astonishingly well built, with all the
improvements that hundreds of thousands of generations of experience
have been able to suggest, but it is self-repairing, self-cleaning, and
self-improving. It never lets itself get out of date. If only given an
adequate supply of fuel and water and not driven too hard, it will stand
an astonishing amount of knocking about in all kinds of weather,
repairing itself and recharging its batteries every night, supplying its
own oil, its own paint and polish, and even regulating its own changes
of gear, according to the nature of the work it has to do. Simply as an
endurance racer it is the toughest and longest-winded thing on earth
and can run down and tire out every paw, pad, or hoof that strikes the
ground--wolf, deer, horse, antelope, wild goat. This is only a sample of
its toughness and resisting power all along the line.

These wide powers of self-support and adjustment overbalance a hundred
times any little remnant defects in its machinery or gearing. Easily
ninety-nine per cent of all our troubl