tence. It appears to me that the power of domestication resolves
itself into the accumulated effects of a change of all or some of the
natural conditions of the life of the species, often associated with
excess of food. These conditions moreover, I may add, can seldom remain,
owing to the mutability of the affairs, habits, migrations, and
knowledge of man, for very long periods the same. I am the more inclined
to come to this conclusion from finding, as we shall hereafter show,
that changes of the natural conditions of existence seem peculiarly to
affect the action of the reproductive system{214}. As we see that
hybrids and mongrels, after the first generation, are apt to vary much,
we may at least conclude that variability does not altogether depend on
excess of food.

{210} See _Origin_, Ed. i. p. 7, vi. p. 7.

{211} <Note in the original.> "Isidore G. St Hilaire insists that
breeding in captivity essential element. Schleiden on alkalies.
<See _Var. under Dom._, Ed. ii. vol. II. p. 244, note 10.> What is
it in domestication which causes variation?"

{212} <Note in the original.> "It appears that slight changes of
condition <are> good for health; that more change affects the
generative system, so that variation results in the offspring;
that still more change checks or destroys fertility not of the
offspring." Compare the _Origin_, Ed. i. p. 9, vi. p. 11. What the
meaning of "not of the offspring" may be is not clear.

{213} In the _Origin_, Ed. i. p. 41, vi. p. 46 the question is
differently treated; it is pointed out that a large stock of
individuals gives a better chance of available variations
occurring. Darwin quotes from Marshall that sheep in small lots can
never be improved. This comes from Marshall's _Review of the
Reports to the Board of Agriculture_, 1808, p. 406. In this Essay
the name Marshall occurs in the margin. Probably this refers to
_loc. cit._ p. 200, where unshepherded sheep in many parts of
England are said to be similar owing to mixed breeding not being
avoided.

{214} See _Origin_, Ed. i. p. 8, vi. p. 8.

After these views, it may be asked how it comes that certain animals
and plants, which have been domesticated for a considerable length of
time, and transported from very different conditions of existence, have
not varied much, or scarcely at all; for instance, the ass, peacock,