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.