emarkable secondary sexual characters. 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; I particularly attended
to Mr. Waterhouse's remark, whilst investigating this order, and I am
fully convinced that the rule almost always holds good. I shall, in a
future work, give a list of all the more remarkable cases. I will here
give only 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 distinct 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 the same species is so great that it is no exaggeration
to state that the varieties of the same species differ more from each
other in the characters derived from these important organs, than do the
species belonging to other distinct genera.

As with birds the individuals of the same species, inhabiting the same
country, vary extremely little, I have particularly attended to them;
and the rule certainly seems to hold good in this class. I cannot make
out that it applies to plants, and this would have seriously 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 a species, the fair presumption is that it is of high importance
to that species: nevertheless it is in this case 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 are descended
from some other species, and have been modified through natural
selection, I think we can obtain some light. First let me make some
preliminary remarks. If, in our domestic animals, any part or the
whole animal be neglected, and no selection