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 {150} 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 remarkab