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.
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