r functions; and as long as the same
part has to perform diversified work, we can perhaps see why it
should remain variable, that is, why natural selection should not have
preserved or rejected each little deviation of form so carefully as when
the part has to serve for some one special purpose. In the 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 purpose must be of some particular
shape. Natural selection, it should never be forgotten, can act solely
through and for the advantage of each being.

Rudimentary parts, as is generally admitted, are apt to be highly
variable. We shall have to recur to this subject; and I will here only
add that their variability seems to result from their uselessness,
and consequently from natural selection having had no power to check
deviations in their structure.

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 by a remark to the above effect made
by Mr. Waterhouse. Professor Owen, also, seems to have come to a nearly
similar conclusion. It is hopeless to attempt to convince any one of the
truth of the above 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
allowances 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 one species or in a few species in comparison with the same
part in many closely allied species. Thus, the wing of the bat is a most
abnormal structure in the class of mammals; but the rule would not apply
here, because the whole group of bats possesses wings; it would apply
only if some one species had wings developed in a 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, relates 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 more rarely to females, as they seldom offer
r