own proper exciting cause. Although we can
seldom trace the precise relation between cause and effect, yet the
considerations presently to be given lead to the conclusion that each
modification must have its own distinct cause. When we hear of an infant
born, for instance, with a crooked finger, a misplaced tooth, or other
slight deviation of structure, it is difficult to bring the conviction home
to the mind that such abnormal cases are the result of fixed laws, and not
of what we blindly call accident. Under this point of view the following
case, which has been carefully examined and communicated to me by Dr.
William Ogle, is highly instructive. Two girls, born as twins, and in all
respects extremely alike, had their little fingers on both hands crooked;
and in both children the second bicuspid tooth in the upper jaw, of the
second dentition, was misplaced; for these teeth, instead of standing in a
line with the others, grew from the roof of the mouth behind the first
bicuspids. Neither the parents nor any other member of the family had
exhibited any similar peculiarity. Now, as both these children were
affected in exactly the same manner by both deviations of structure, the
idea of accident is at once excluded; and we are compelled to admit that
there must have existed some precise and sufficient cause which, if it had
occurred a hundred times, would have affected a hundred children.

We will now consider the general arguments, which appear to me to have
great weight, in favour of the view that variations of all kinds and
degrees are directly or indirectly caused by the conditions of life to
which each being, and more especially its ancestors, have been exposed.

No one doubts that domesticated productions are more variable than organic
beings which have never been removed from their {254} natural conditions.
Monstrosities graduate so insensibly into mere variations that it is
impossible to separate them; and all those who have studied monstrosities
believe that they are far commoner with domesticated than with wild animals
and plants;[605] and in the case of plants, monstrosities would be equally
noticeable in the natural as in the cultivated state. Under nature, the
individuals of the same species are exposed to nearly uniform conditions,
for they are rigorously kept to their proper places by a host of competing
animals and plants; they have, also, long been habituated to their
conditions of life; but it cannot be s