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 be applied, that part (for instance, the comb
in the Dorking fowl) or the whole breed will cease to have a uniform
character: and the breed may be said to be degenerating. In rudimentary
organs, and in those which have been but little specialised for any
particular purpose, and perhaps in polymorphic groups, we see a nearly
parallel case; for in such cases natural selection either has not or cannot
come into full play, and thus the organisation is left in a fluctuating
condition.
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