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 difficulties in the way of the theory.

It is to show that such difficulties do not exist, and in the full conviction that an adequate knowledge of the facts of variation affords the only sure foundation for the Darwinian theory of the origin of species, that this chapter has been written.
FOOTNOTES: [Footnote 16: _Foraminifera_, preface, p.

x.] [Footnote 17: _United States Geological Survey of the Territories_, 1874.] [Footnote 18: _Proceedings of the Entomological Society of London_, 1875, p.

vii.] [Footnote 19: _Ann.