large in its
amount and affecting, not a few, but considerable masses of the
individuals which make up the species, will be to many entirely new.
Equally important is the fact that the variability extends to every
organ and every structure, external and internal; while perhaps most
important of all is the independent variability of these several parts,
each one varying without any constant or even usual dependence on, or
correlation with, other parts. No doubt there is some such correlation
in the differences that exist between species and species--more
developed wings usually accompanying smaller feet and _vice versa_--but
this is, generally, a useful adaptation which has been brought about by
natural selection, and does not apply to the individual variability
which occurs within the species.

It is because these facts of variation are so important and so little
understood, that they have been discussed in what will seem to some
readers wearisome and unnecessary detail. Many naturalists, however,
will hold that even more evidence is required; and more, to almost any
amount, could easily have been given. The character and variety of that
already adduced will, however, I trust, convince most readers that the
facts are as stated; while they have been drawn from a sufficiently wide
area to indicate a general principle throughout nature.

If, now, we fully realise these facts of variation, along with those of
rapid multiplication and the struggle for existence, most of the
difficulties in the way of comprehending how species have originated
through natural selection will disappear. For whenever, through changes
of climate, or of altitude, or of the nature of the soil, or of the area
of the country, any species are exposed to new dangers, and have to
maintain themselves and provide for the safety of their offspring under
new and more arduous conditions, then, in the variability of all parts,
organs, and structures, no less than of habits and intelligence, we have
the means of producing modifications which will certainly bring the
species into harmony with its new conditions. And if we remember that
all such physical changes are slow and gradual in their operation, we
shall see that the amount of variation which we know occurs in every new
generation will be quite sufficient to enable modification and
adaptation to go on at the same rate. Mr. Darwin was rather inclined to
exaggerate the necessary slowness of the action of n