erate one of them unduly,
or fail to rectify it by a rebound oscillation, and you have disease.

Or it is like the children's game of shuttlecock. So long as the flying
shuttle keeps moving in its restless course to and fro, life is. A
single stop is death. The very same blow which, rightly placed, sends it
like an arrow to the safe centre of the opposing racket, if it fall
obliquely, or even with too great or too little force, drives it
perilously wide of its mark. It can recover the safe track only by a
sudden and often violent lunge of the opposing racket. The straight
course is life, the tangent disease, the saving lunge recovery.

One and the same force produces all.

In the millions of tiny blows dealt every minute in our body-battle,
what wonder if some go wide of the mark!

CHAPTER II

OUR LEGACY OF HEALTH: THE POWER OF HEREDITY IN THE PREVENTION OF DISEASE

The evil in things always bulks large in our imaginations. It is no mere
coincidence that the earliest gods of a race are invariably demons. Our
first conception of the great forces of nature is that they are our
enemies. This misconception is not only natural, but even necessary on
the sternest of physical bases. The old darky, Jim, in Huckleberry Finn,
hit upon a profound and far-reaching truth when he replied in answer to
Huck's question whether among all the signs and portents with which his
mind was crammed--like black cats and seeing the moon over your left
shoulder and "harnts"--some were not indications of good luck instead of
all being of evil omen:--

"Mighty few--an' _dey_ ain't no use to a body. What fur you want to know
when good luck's a-comin'? Want to keep it off?"

It isn't the good, either in the forces of nature or in our fellows,
that keeps us watchful, but the evil. Hence our proneness to declare in
all ages that evil is stronger than good and that "all men are liars."
One injury done us by storm, by sunstroke, by lightning-flash, will make
a more lasting impression upon our memories than a thousand benefits
conferred by these same forces. Besides, evil has to be sharply looked
out for and guarded against. Well enough can be safely let alone.

The conviction is steadily growing, among both physicians and
biologists, that this attitude has caused a serious, if not vital,
misconception of the influence of that great conservative and
preservative force of nature--heredity. We hear a great deal of
hereditary disease, heredi