markable 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 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 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 part
(