this
variability extends, so far as we know, to every part and organ, whether
external or internal, as well as to every mental faculty. Yet more
important is the fact that each part or organ varies to a considerable
extent independently of other parts. Again, we have shown, by abundant
evidence, that the variation that occurs is very large in
amount--usually reaching 10 or 20, and sometimes even 25 per cent of the
average size of the varying part; while not one or two only, but from 5
to 10 per cent of the specimens examined exhibit nearly as large an
amount of variation. These facts have been brought clearly before the
reader by means of numerous diagrams, drawn to scale and exhibiting the
actual variations in inches, so that there can be no possibility of
denying either their generality or their amount. The importance of this
full exposition of the subject will be seen in future chapters, when we
shall frequently have to refer to the facts here set forth, especially
when we deal with the various theories of recent writers and the
criticisms that have been made of the Darwinian theory.

A full exposition of the facts of variation among wild animals and
plants is the more necessary, because comparatively few of them were
published in Mr. Darwin's works, while the more important have only been
made known since the last edition of _The Origin of Species_ was
prepared; and it is clear that Mr. Darwin himself did not fully
recognise the enormous amount of variability that actually exists. This
is indicated by his frequent reference to the extreme slowness of the
changes for which variation furnishes the materials, and also by his use
of such expressions as the following: "A variety when once formed must
again, _perhaps after a long interval of time_, vary or present
individual differences of the same favourable nature as before"
(_Origin_, p. 66). And again, after speaking of changed conditions
"affording a better chance of the occurrence of favourable variations,"
he adds: "_Unless such occur natural selection can do nothing_"
(_Origin_, p. 64). These expressions are hardly consistent with the fact
of the constant and large amount of variation, of every part, in all
directions, which evidently occurs in each generation of all the more
abundant species, and which must afford an ample supply of favourable
variations whenever required; and they have been seized upon and
exaggerated by some writers as proofs of the extreme dif