e same way that a knife which has to cut
all sorts of things may be of almost any shape; whilst a tool for
some particular object had better be of some particular shape. Natural
selection, it should never be forgotten, can act on each part of each
being, solely through and for its advantage.

Rudimentary parts, it has been stated by some authors, and I believe
with truth, are apt to be highly variable. We shall have to recur to the
general subject of rudimentary and aborted organs; and I will here only
add that their variability seems to be owing to their uselessness, and
therefore to natural selection having no power to check deviations in
their structure. Thus rudimentary parts are left to the free play of the
various laws of growth, to the effects of long-continued disuse, and to
the tendency to reversion.

A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR MANNER,
IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES, TENDS TO BE HIGHLY
VARIABLE.

Several years ago I was much struck with a remark, nearly to the above
effect, published by Mr. Waterhouse. I infer also from an observation
made by Professor Owen, with respect to the length of the arms of the
ourang-outang, that he has come to a nearly similar conclusion. It is
hopeless to attempt to convince any one of the truth of this proposition
without giving the long array of facts which I have collected, and which
cannot possibly be here introduced. I can only state my conviction that
it is a rule of high generality. I am aware of several causes of
error, but I hope that I have made due allowance for them. It should
be understood that the rule by no means applies to any part, however
unusually developed, unless it be unusually developed in comparison with
the same part in closely allied species. Thus, the bat's wing is a most
abnormal structure in the class mammalia; but the rule would not here
apply, because there is a whole group of bats having wings; it would
apply only if some one species of bat had its wings developed in some
remarkable manner in comparison with the other species of the same
genus. The rule applies very strongly in the case of secondary sexual
characters, when displayed in any unusual manner. The term, secondary
sexual characters, used by Hunter, applies to characters which are
attached to one sex, but are not directly connected with the act of
reproduction. The rule applies to males and females; but as females more
rarely offer re