one animal produced by man's selection; while
we think little of the diversities of the fox and the wolf, or the horse
and the zebra, because we have been accustomed to look upon them as
radically distinct animals, not as the results of nature's selection of
the varieties of a common ancestor.

FOOTNOTES:

[Footnote 31: Darwin, _Animals and Plants under Domestication_, vol. i.
p. 322.]

[Footnote 32: These facts are taken from Darwin's _Domesticated Animals
and Cultivated Plants_, vol. i. pp. 359, 360, 392-401; vol. ii. pp. 231,
275, 330.]

[Footnote 33: See Darwin's _Animals and Plants under Domestication_,
vol. i. pp. 40-42.]

[Footnote 34: Mr. Brent in _Journal of Horticulture_, 1861, p. 76;
quoted by Darwin, _Animals and Plants under Domestication_, vol. i. p.
151.]

[Footnote 35: This account of domestic pigeons is greatly condensed from
Mr. Darwin's work already referred to.]

[Footnote 36: _Animals and Plants under Domestication_, vol. ii. pp.
307-311.]

CHAPTER V

NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST

Effect of struggle for existence under unchanged conditions--The
effect under change of conditions--Divergence of character--In
insects--In birds--In mammalia--Divergence leads to a maximum of
life in each area--Closely allied species inhabit distinct
areas--Adaptation to conditions at various periods of life--The
continued existence of low forms of life--Extinction of low
types among the higher animals--Circumstances favourable to the
origin of new species--Probable origin of the dippers--The
importance of isolation--On the advance of organisation by
natural selection--Summary of the first five chapters.

In the preceding chapters we have accumulated a body of facts and
arguments which will enable us now to deal with the very core of our
subject--the formation of species by means of natural selection. We have
seen how tremendous is the struggle for existence always going on in
nature owing to the great powers of increase of all organisms; we have
ascertained the fact of variability extending to every part and organ,
each of which varies simultaneously and for the most part independently;
and we have seen that this variability is both large in its amount in
proportion to the size of each part, and usually affects a considerable
proportion of the individuals in the large and dominant species. And,
lastly, we have seen how simi