plant, we may look at the result as probably due to the selection of
so-called spontaneous variations. Every one knows that hard work thickens
the epidermis on the hands; and when we hear that with infants long before
their birth the epidermis is thicker on the palms and soles of the feet
than on any other part of the body, as was observed with admiration by
Albinus,[733] we are naturally inclined to attribute this to the inherited
effects of long-continued use or pressure. We are tempted to extend the
same view even to the hoofs of quadrupeds; but who will pretend to
determine how far natural selection may have aided in the formation of
structures of such obvious importance to the animal?

That use strengthens the muscles may be seen in the limbs of artisans
who follow different trades; and when a muscle is strengthened, the
tendons, and the crests of bone to which they are attached, become
enlarged; and this must likewise be the case with the blood-vessels and
nerves. On the other hand, when a limb is not used, as by Eastern
fanatics, or when the nerve supplying it with nervous power is
effectually destroyed, the muscles wither. So again, when the eye is
destroyed the optic nerve becomes atrophied, sometimes even in the
course of a few months.[734] The Proteus is furnished with branchiae as
well as with lungs: and Schreibers[735] found that when the animal was
compelled to live in deep water the branchiae were developed to thrice
their ordinary size, and the lungs were partially atrophied. When, on
the other hand, the animal was compelled to live in shallow water, the
lungs became larger and more vascular, whilst the branchiae disappeared
in a more or less complete degree. Such modifications as these are,
however, of comparatively little value for us, as we do not actually
know that they tend to be inherited.

In many cases there is reason to believe that the lessened use of
various organs has affected the corresponding parts in the offspring.
But there is no good evidence that this ever follows in the course of a
single generation. It appears, as in the case of general or indefinite
variability, that several generations must be subjected to changed
habits for any appreciable result. Our domestic fowls, ducks, and geese
have almost lost, not {298} only in the individual but in the race,
their power of flight; for we do