atural selection;
but with the knowledge we now possess of the great amount and range of
individual variation, there seems no difficulty in an amount of change,
quite equivalent to that which usually distinguishes allied species,
sometimes taking place in less than a century, should any rapid change
of conditions necessitate an equally rapid adaptation. This may often
have occurred, either to immigrants into a new land, or to residents
whose country has been cut off by subsidence from a larger and more
varied area over which they had formerly roamed. When no change of
conditions occurs, species may remain unchanged for very long periods,
and thus produce that appearance of stability of species which is even
now often adduced as an argument against evolution by natural selection,
but which is really quite in harmony with it.

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.

In the presen