ce.
[Illustration: FIG. 3.--Variation of Wings and Tail.]
There is here no question of "minute" or "infinitesimal" variation,
which many people suppose to be the only kind of variation that exists.
It cannot even be called small; yet from all the evidence we now possess
it seems to be the amount which characterises most of the common species
of birds.
It may be said, however, that these are the extreme variations, and only
occur in one or two individuals, while the great majority exhibit little
or no difference. Other diagrams will show that this is not the case;
but even if it were so, it would be no objection at all, because these
are the extremes among thirty specimens only. We may safely assume that
these thirty specimens, taken by chance, are not, in the case of all
these species, exceptional lots, and therefore we might expect at least
two similarly varying specimens in each additional thirty. But the
number of individuals, even in a very rare species, is probably thirty
thousand or more, and in a common species thirty, or even three hundred,
millions. Even one individual in each thirty, varying to the amount
shown in the diagram, would give at least a million in the total
population of any common bird, and among this million many would vary
much more than the extreme among thirty only. We should thus have a vast
body of individuals varying to a large extent in the length of the wings
and tail, and offering ample material for the modification of these
organs by natural selection. We will now proceed to show that other
parts of the body vary, simultaneously, but independently, to an equal
amount.
[Illustration: FIG. 4.--Dolichonyx oryzivorus. 20 Males.]
[Illustration: FIG. 5.--Agelaeus phoeniceus. 40 Males.]
The first bird taken is the common Bob-o-link or Rice-bird (Dolichonyx
oryzivorus), and the Diagram, Fig. 4, exhibits the variations of seven
important characters in twenty male adult specimens.[21] These
characters are--the lengths of the body, wing, tail, tarsus, middle toe,
outer toe, and hind toe, being as many as can be conveniently exhibited
in one diagram. The length of the body is not given by Mr. Allen, but as
it forms a convenient standard of comparison, it has been obtained by
deducting the length of the tail from the total length of the birds as
given by him. The diagram has been constructed as follows:--The twenty
specimens are first arranged in a series according to the body-lengths
(
|