oint to the general conclusion we have now reached,
and afford us a not unsatisfactory solution of the great problem of
hybridism in relation to the origin of species by means of natural
selection. Further experimental research is needed in order to complete
the elucidation of the subject; but until these additional facts are
forthcoming no new theory seems required for the explanation of the
phenomena.

FOOTNOTES:

[Footnote 51: Darwin's _Animals and Plants under Domestication_, vol.
ii. pp. 163-170.]

[Footnote 52: For a full account of these interesting facts and of the
various problems to which they give rise, the reader must consult
Darwin's volume on _The Different Forms of Flowers in Plants of the same
Species_, chaps, i.-iv.]

[Footnote 53: See _Nature_, vol. xxi. p. 207.]

[Footnote 54: Low's _Domesticated Animals of Great Britain_,
Introduction, p. lxiv.]

[Footnote 55: Low's _Domesticated Animals_, p. 28.]

[Footnote 56: _Amaryllidaceae_, by the Hon. and Rev. William Herbert, p.
379.]

[Footnote 57: _Origin of Species_, p. 239.]

[Footnote 58: _Origin of Species_, sixth edition, p. 9.]

[Footnote 59: In the _Medico-Chirurgical Transactions_, vol. liii.
(1870), Dr. Ogle has adduced some curious physiological facts bearing on
the presence or absence of white colours in the higher animals. He
states that a dark pigment in the olfactory region of the nostrils is
essential to perfect smell, and that this pigment is rarely deficient
except when the whole animal is pure white, and the creature is then
almost without smell or taste. He observes that there is no proof that,
in any of the cases given above, the black animals actually eat the
poisonous root or plant; and that the facts are readily understood if
the senses of smell and taste are dependent on a pigment which is absent
in the white animals, who therefore eat what those gifted with normal
senses avoid. This explanation however hardly seems to cover the facts.
We cannot suppose that almost all the sheep in the world (which are
mostly white) are without smell or taste. The cutaneous disease on the
white patches of hair on horses, the special liability of white terriers
to distemper, of white chickens to the gapes, and of silkworms which
produce yellow silk to the fungus, are not explained by it. The
analogous facts in plants also indicate a real constitutional relation
with colour, not an affection of the sense of smell and taste only.]

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