On the principles, and by the light of the facts, now briefly
summarised, we have been able, in the present chapter, to indicate how
natural selection acts, how divergence of character is set up, how
adaptation to conditions at various periods of life has been effected,
how it is that low forms of life continue to exist, what kind of
circumstances are most favourable to the formation of new species, and,
lastly, to what extent the advance of organisation to higher types is
produced by natural selection. We will now pass on to consider some of
the more important objections and difficulties which have been advanced
by eminent naturalists.
FOOTNOTES:
[Footnote 37: _Origin of Species_, p. 71.]
[Footnote 38: Yarrell's _British Birds_, fourth edition, vol. iii. p.
77.]
[Footnote 39: _Origin of Species_, p. 89.]
[Footnote 40: _Nature_, vol. xxx. p. 30.]
CHAPTER VI
DIFFICULTIES AND OBJECTIONS
Difficulty as to smallness of variations--As to the right
variations occurring when required--The beginnings of important
organs--The mammary glands--The eyes of flatfish--Origin of the
eye--Useless or non-adaptive characters--Recent extension of the
region of utility in plants--The same in animals--Uses of
tails--Of the horns of deer--Of the scale-ornamentation of
reptiles--Instability of non-adaptive characters--Delboeuf's
law--No "specific" character proved to be useless--The swamping
effects of intercrossing--Isolation as preventing
intercrossing--Gulick on the effects of isolation--Cases in
which isolation is ineffective.
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