of domestic
animals--Domestic pigeons--Acclimatisation--Circumstances
favourable to selection by man--Conditions favourable to
variation--Concluding remarks

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

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

CHAPTER VII

ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL
STERILITY OF THEIR HYBRID OFFSPRING

Statement of the problem--Extreme susceptibility of the
reproductive functions--Reciprocal crosses--Individual
differences in respect to cross-fertilisation--Dimorphism and
trimorphism among plants--Cases of the fertility of hybrids and
of the infertility of mongrels--The effects of close
interbreeding--Mr. Huth's objections--Fertile hybrids among
animals--Fertility of hybrids among plants--Cases of sterility
of mongrels--Parallelism between crossing and change of
conditions--Remarks on the facts of hybridity--Sterility due to
changed conditions and usually correlated with other
characters--Correlation of colour with constitutional
peculiari