the peach and nectarine, each with their many
varieties, might have been introduced. No doubt the relationship of our
different domestic breeds has been obscured in an extreme degree by
their crossing; and likewise from the slight difference between many
breeds it has probably often happened that a "sport" from one breed has
less closely resembled its parent breed than some other breed, and has
therefore been classed with the latter. Moreover the effects of a
similar climate{446} may in some cases have more than counterbalanced
the similarity, consequent on a common descent, though I should think
the similarity of the breeds of cattle of India or sheep of Siberia was
far more probably due to the community of their descent than to the
effects of climate on animals descended from different stocks.

{446} A general statement of the influence of conditions on
variation occurs in the _Origin_, Ed. i. pp. 131-3, vi. pp. 164-5.

Notwithstanding these great sources of difficulty, I apprehend every
one would admit, that if it were possible, a genealogical classification
of our domestic varieties would be the most satisfactory one; and as far
as varieties were concerned would be the natural system: in some cases
it has been followed. In attempting to follow out this object a person
would have to class a variety, whose parentage he did not know, by its
external characters; but he would have a distinct ulterior object in
view, namely, its descent in the same manner as a regular systematist
seems also to have an ulterior but undefined end in all his
classifications. Like the regular systematist he would not care whether
his characters were drawn from more or less important organs as long as
he found in the tribe which he was examining that the characters from
such parts were persistent; thus amongst cattle he does value a
character drawn from the form of the horns more than from the
proportions of the limbs and whole body, for he finds that the shape of
the horns is to a considerable degree persistent amongst cattle{447},
whilst the bones of the limbs and body vary. No doubt as a frequent rule
the more important the organ, as being less related to external
influences, the less liable it is to variation; but he would expect that
according to the object for which the races had been selected, parts
more or less important might differ; so that characters drawn from parts
generally most liable to vary, as colour, might in some