quence of new organization appear from variety of facts in the
more mature animals; thus ... the lungs must be previously formed before
their exertions to obtain fresh air can exist; the throat, or
oesophagus, must be formed previous to the sensation or appetites of
hunger and thirst, one of which seems to reside at the upper end and the
other at the lower end of that canal."[173]

It seems to me Dr. Darwin is wrong in supposing that the organ must have
preceded the power to use it. The organ and its use--the desire to do
and the power to do--have always gone hand in hand, the organism finding
itself able to do more according as it advanced its desires, and
desiring to do more simultaneously with any increase in power, so that
neither appetency nor organism can claim precedence, but power and
desire must be considered as Siamese twins begotten together, conceived
together, born together, and inseparable always from each other. At the
same time they are torn by mutual jealousy; each claims, with some vain
show of reason, to have been the elder brother; each intrigues
incessantly from the beginning to the end of time to prevent the other
from outstripping him; each is in turn successful, but each is doomed to
death with the extinction of the other.

"So inflamed tendons and membranes, and even bones, acquire new
sensations; and the parts of mutilated animals, as of wounded snails and
polypi and crabs, are reproduced; and at the same time acquire
sensations adapted to their situation. Thus when the head of a snail is
reproduced after decollation with a sharp razor, those curious
telescopic eyes are also reproduced, and acquire their sensibility to
light, as well as their adapted muscles for retraction on the approach
of injury.

"With every change, therefore, of organic form or addition of organic
parts, I suppose a new kind of irritability or of sensibility to be
produced; such varieties of irritability or of sensibility exist in our
adult state in the glands; every one of which is furnished with an
irritability or a taste or appetency, and a consequent mode of action
peculiar to itself.

"In this manner I conceive the vessels of the jaws to produce those of
the teeth; those of the fingers to produce the nails; those of the skin
to produce the hair; in the same manner as afterwards, about the age of
puberty, the beard and other great changes in the form of the body and
disposition of the mind are produced in consequen