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 remarkable secondary sexual
characters, it applies
more rarely to them. The rule being so plainly applicable in the case
of secondary sexual characters, may be due to the great variability of
these characters, whether or not displayed in any unusual manner of
which fact I think there can be little doubt. But that our rule is not
confined to secondary sexual characters is clearly shown in the case of
hermaphrodite cirripedes; and I may here add, that I particularly
attended to Mr. Waterhouse s remark, whilst investigating this Order,
and I am fully convinced that the rule almost invariably holds good
with cirripedes. I shall, in my future work, give a list of the more
remarkable cases; I will here only briefly give one, as it illustrates
the rule in its largest application. The opercular valves of sessile
cirripedes (rock barnacles) are, in every sense of the word, very
important structures, and they differ extremely little even in
different genera; but in the several species of one genus, Pyrgoma,
these valves present a marvellous amount of diversification: the
homologous valves in the different species being sometimes wholly
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