our-blindness.

But now comes the great difficulty. Let us further suppose that you
two twins marry wives who are also twins born as like as two peas;
and each pair of you has a family. Which of the two batches of children
will tend on the whole to have the stronger legs? Your legs are strong
by use; your brother's are weak by disuse. But do use and disuse make
any difference to the race? That is the theoretical question which,
above all others, complicates and hampers our present-day attempts
to understand heredity.

In technical language, this is the problem of use-inheritance,
otherwise known as the inheritance of acquired characters. It is apt
to seem obvious to the plain man that the effects of use and disuse
are transmitted to offspring. So, too, thought Lamarck, who half a
century before Darwin propounded a theory of the origin of species
that was equally evolutionary in its way. Why does the giraffe have
so long a neck? Lamarck thought it was because the giraffe had acquired
a habit of stretching his neck out. Every time there was a bad season,
the giraffes must all stretch up as high as ever they could towards
the leafy tops of the trees; and the one that stretched up farthest
survived, and handed on the capacity for a like feat to his fortunate
descendants. Now Darwin himself was ready to allow that use and disuse
might have some influence on the offspring's inheritance; but he
thought that this influence was small as compared with the influence
of what, for want of a better term, he called spontaneous variation.
Certain of his followers, however, who call themselves Neo-Darwinians,
are ready to go one better. Led by the German biologist, Weismann,
they would thrust the Lamarckians, with their hypothesis of
use-inheritance, clean out of the field. Spontaneous variation, they
assert, is all that is needed to prepare the way for the selection
of the tall giraffe. It happened to be born that way. In other words,
its parents had it in them to breed it so. This is not a theory that
tells one anything positive. It is merely a caution to look away from
use and disuse to another explanation of variation that is not yet
forthcoming.

After all, the plain man must remember that the effects of use and
disuse, which he seems to see everywhere about him, are mixed up with
plenty of apparent instances to the contrary. He will smile, perhaps,
when I tell him that Weismann cut off the tails of endless mice, and,
breeding