ty of inhabitants
of any small area, and to naturalisation--Action of Natural Selection,
through Divergence of Character and Extinction, on the descendants from a
common parent--Explains the Grouping of all organic beings

80-130

CHAPTER V.

LAWS OF VARIATION.

Effects of external conditions--Use and disuse, combined with natural
selection; organs of flight and of vision--Acclimatisation--Correlation of
growth--Compensation and economy of growth--False correlations--Multiple,
rudimentary, and lowly organised structures variable--Parts developed in an
unusual manner are highly variable: specific characters more variable than
generic: secondary sexual characters variable--Species of the same genus
vary in an analogous manner--Reversions to long-lost characters--Summary

131-170

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CHAPTER VI.

DIFFICULTIES ON THEORY.

Difficulties on the theory of descent with
modification--Transitions--Absence or rarity of transitional
varieties--Transitions in habits of life--Diversified habits in the same
species--Species with habits widely different from those of their
allies--Organs of extreme perfection--Means of transition--Cases of
difficulty--Natura non facit saltum--Organs of small importance--Organs not
in all cases absolutely perfect--The law of Unity of Type and of the
Conditions of Existence embraced by the theory of Natural Selection

171-206

CHAPTER VII.

INSTINCT.

Instincts comparable with habits, but different in their origin--Instincts
graduated--Aphides and ants--Instincts variable--Domestic instincts, their
origin--Natural instincts of the cuckoo, ostrich, and parasitic
bees--Slave-making ants--Hive-bee, its cell-making instinct--Difficulties
on the theory of the Natural Selection of instincts--Neuter or sterile
insects--Summary

207-244

CHAPTER VIII.

HYBRIDISM.

Distinction between the sterility of first crosses and of
hybrids--Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication--Laws governing the sterility of
hybrids--Sterility not a special endowment, but incidental on other
differences--Causes of the sterility of first crosses and of
hybrids--Parallelism between the effects of changed conditions of life and
crossing--Fertility of varieties when crossed and of their mongrel
offspring not universal--Hybrids and mongrels compared independently of
their fertility--Summary

245-278

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CHAPTER IX.

ON THE IMPERFECTION OF