which I am capable, that the view which most naturalists until recently
entertained, and which I formerly entertained--namely, that each species
has been independently created--is erroneous. I am fully convinced that
species are not immutable; but that those belonging to what are called
the same genera are lineal descendants of some other and generally
extinct species, in the same manner as the acknowledged varieties of
any one species are the descendants of that species. Furthermore, I am
convinced that natural selection has been the most important, but not
the exclusive, means of modification.

CHAPTER I. VARIATION UNDER DOMESTICATION.

Causes of Variability--Effects of Habit and the use and disuse of
Parts--Correlated Variation--Inheritance--Character of Domestic
Varieties--Difficulty of distinguishing between Varieties and
Species--Origin of Domestic Varieties from one or more Species--Domestic
Pigeons, their Differences and Origin--Principles of Selection,
anciently followed, their Effects--Methodical and Unconscious
Selection--Unknown Origin of our Domestic Productions--Circumstances
favourable to Man's power of Selection.

CAUSES OF VARIABILITY.

When we compare the individuals of the same variety or sub-variety of
our older cultivated plants and animals, one of the first points which
strikes us is, that they generally differ more from each other than do
the individuals of any one species or variety in a state of nature. And
if we reflect on the vast diversity of the plants and animals which have
been cultivated, and which have varied during all ages under the most
different climates and treatment, we are driven to conclude that this
great variability is due to our domestic productions having been raised
under conditions of life not so uniform as, and somewhat different from,
those to which the parent species had been exposed under nature. There
is, also, some probability in the view propounded by Andrew Knight, that
this variability may be partly connected with excess of food. It seems
clear that organic beings must be exposed during several generations to
new conditions to cause any great amount of variation; and that, when
the organisation has once begun to vary, it generally continues varying
for many generations. No case is on record of a variable organism
ceasing to vary under cultivation. Our oldest cultivated plants, such
as wheat, still yield new varieties: our oldest domesticated anim