of its construction is a structural adaptation; and when such
adaptations are given the mechanist has for the most part a relatively
easy task in his interpretation. He has a far more difficult knot to
disentangle in the case of the so-called functional adaptations, where
the organism modifies its activities (and often also its structure) in
response to changed conditions. The nature of these phenomena may be
illustrated by a few examples so chosen as to form a progressive
series. If a spot on the skin be rubbed for some time the first result
is a direct and obviously mechanical one; the skin is worn away. But
if the rubbing be continued long enough, and is not too severe, an
indirect effect is produced that is precisely the opposite of the
initial direct one; the skin is replaced, becomes thicker than before,
and a callus is produced that protects the spot from further injury.
The healing of a wound involves a similar action. Again, remove one
kidney or one lung and the remaining one will in time enlarge to
assume, as far as it is able, the functions of both. If the leg of a
salamander or a lobster be amputated, the wound not only heals but a
new leg is regenerated in place of that which has been lost. If a
flatworm be cut in two, the front piece grows out a new tail, the hind
piece a new head, and two perfect worms result. Finally, it has been
found in certain cases, including animals as highly organized as
salamanders, that if the egg be separated into two parts at an early
period of development each part develops into a perfect embryo animal
of half the usual size, and a pair of twins results. In each of these
cases the astonishing fact is that a mechanical injury sets up in the
organism a complicated adaptive response in the form of operations
which in the end counteract the initial mechanical effect. It is no
doubt true that somewhat similar self-adjustments or responses may be
said to take place in certain non-living mechanical systems, such as
the spinning top or the gyroscope; but those that occur in the living
body are of such general occurrence, of such complexity and variety,
and of so design-like a quality, that they may fairly be regarded as
among the most characteristic of the vital activities. It is precisely
this characteristic of many vital phenomena that renders their
accurate analysis so difficult and complex a task; and it is largely
for this reason that the biological sciences, as a whole, still st