le secondary sexual characters, it
applies {151} 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 unlike in shape; and the amount of variation in the
individuals of several of the species is so great, that it is no
exaggeration to state that the varieties differ more from each other in the
characters of these important valves than do other species of distinct
genera.

As birds within the same country vary in a remarkably small degree, I have
particularly attended to them, and the rule seems to me certainly to hold
good in this class. I cannot make out that it applies to plants, and this
would seriously have shaken my belief in its truth, had not the great
variability in plants made it particularly difficult to compare their
relative degrees of variability.

When we see any part or organ developed in a remarkable degree or manner in
any species, the fair {152} presumption is that it is of high importance to
that species; nevertheless the part in this case is eminently liable to
variation. Why should this be so? On the view that each species has been
independently created, with all its parts as we now see them, I can see no
explanation. But on the view that groups of species have descended from
other species, and have been modified through natural selection, I think we
can obtain some light. In our domestic animals, if any part, or the whole
animal, be neglected and no selection be applied, that p