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 (for instance, the comb in the Dorking fowl) or the whole breed
will cease to have a nearly uniform character. The breed will then be
said to have degenerated. 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 natural 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. But what here more especially concerns us is, that in our
domestic animals those points, which at the present time are undergoing
rapid change by continued selection, are also eminently liable to
variation. Look at the breeds of the pigeon; see what a prodigious
amount of difference there is in the beak of the different tumblers, in
the beak and wattle of the different carriers, in the carriage and tail
of our fantails, etc., these being the points now mainly attended to by
English fanciers. Even in the sub-breeds, as in the short-faced
tumbler, it is notoriously difficult to breed them nearly to
perfection, and frequently individuals are born which depart widely
from the standard. There may be truly said to be a constant struggle
119