that the promptings of desires
or wants produced growths of the parts subserving them, accepted the
single _vera causa_ assigned by these writers--the modification of
structures resulting from modification of functions. They recognized
as the sole process in organic development, the adaptation of parts and
powers consequent on the effects of use and disuse--that continual
moulding and re-moulding of organisms to suit their circumstances, which
is brought about by direct converse with such circumstances.

But while this cause accepted by these few is a true cause, since
unquestionably during the life of the individual organism changes of
function produce changes of structure; and while it is a tenable
hypothesis that changes of structure so produced are inheritable; yet it
was manifest to those not prepossessed, that this cause cannot with
reason be assigned for the greater part of the facts. Though in plants
there are some characters which may not irrationally be ascribed to the
direct effects of modified functions consequent on modified
circumstances, yet the majority of the traits presented by plants are
not to be thus explained. It is impossible that the thorns by which a
briar is in large measure defended against browsing animals, can have
been developed and moulded by the continuous exercise of their
protective actions; for in the first place, the great majority of the
thorns are never touched at all, and, in the second place, we have no
ground whatever for supposing that those which are touched are thereby
made to grow, and to take those shapes which render them efficient.
Plants which are rendered uneatable by the thick woolly coatings of
their leaves, cannot have had these coatings produced by any process of
reaction against the action of enemies; for there is no imaginable
reason why, if one part of a plant is eaten, the rest should thereafter
begin to develop the hairs on its surface. By what direct effect of
function on structure, can the shell of a nut have been evolved? Or how
can those seeds which contain essential oils, rendering them unpalatable
to birds, have been made to secrete such essential oils by these actions
of birds which they restrain? Or how can the delicate plumes borne by
some seeds, and giving the wind power to waft them to new stations, be
due to any immediate influences of surrounding conditions? Clearly in
these and in countless other cases, change of structure cannot have been
directly