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3 3X8 4 9

Causes of Variability - Effects of Habit - Correlated Variation
Inheritance Character of Domestic Varieties - Difficulty of
distinguishing between Varieties and Species-Origin of Domestic
Varieties from one or more Species-Domestic Pigeons, their
Differences and Origin — Principles of Selection anciently fol-
lowed, their Effects- - Methodical and Unconscious Selection-
Unknown Origin of our Domestic Productions - Circumstances
favourable to Man's power of Selection

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7-47

CHAPTER II.

VARIATION UNDER NATURE.

Variability Individual differences

Doubtful species - Wide-
ranging, much-diffused, and common species vary most Species
of the larger genera in each country vary more frequently than
the species of the smaller genera Many of the species of the
larger genera resemble varieties in being very closely, but un-
equally, related to each other, and in having restricted ranges

48-70

CHAPTER III.

STRUGGLE FOR EXISTENCE.

Its bearing on natural selection - The term used in a wide sense —
Geometrical ratio of increase Rapid increase of naturalised
animals and plants — Nature of the checks to increase — Compe-
tition universal-Effects of Climate Protection from the
number of individuals - Complex relations of all animals and
plants throughout nature-Struggle for life most severe between
individuals and varieties of the same species; often severe be-
tween species of the same genus - The relation of organism to
organism the most important of all relations

CHAPTER IV.

Page 71-90

NATURAL SELECTION, OR THE SURVIVAL OF THE FITTEST.
Natural Selection-its power compared with man's selection - its
power on characters of trifling importance- its power at all ages
and on both sexes - Sexual Selection On the generality of
intercrosses between individuals of the same species — Circum-
stances favourable and unfavourable to the results of Natural
Selection, namely, intercrossing, isolation, number of individuals
- Slow action Extinction caused by Natural Selection —
Divergence of Character, related to the diversity 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 - Advance in organisation - Low forms
preserved Objections considered Uniformity of certain cha-
racters due to their unimportance, and to their not having been
acted on by Natural Selection Indefinite multiplication of

-

-

91-164

CHAPTER V.

LAWS OF VARIATION.

w

Acclimati-

Effects of changed conditions - Use and disuse, combined with
Natural Selection; organs of flight and of vision
sation Correlated variation-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
Page 165-206

CHAPTER VI.

DIFFICULTIES OF THE THEORY.

-

Difficulties of 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-Modes of transition Cases of
difficulty - Natura non facit saltum - Organs of small import-
ance Orgaus 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
.. 207-254

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 Changes of instinct and structure not
necessarily simultaneous - Difficulties of the theory of the
Natural Selection of instincts-Neuter or sterile insects —
Summary
. 255-298

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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, not accumulated by natural
selection - Causes of the sterility of first crosses and of hybrids
- Parallelism between the effects of changed conditions of life
and of crossing- Dimorphism and Trimorphism - Fertility of
varieties when crossed and of their mongrel offspring not uni-
versal — Hybrids and mongrels compared independently of their
fertility Summary
.. Page 299-344

CHAPTER IX.

ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

On the absence of intermediate varieties at the present day - On
the nature of extinct intermediate varieties; on their number-
On the lapse of time, as inferred from the rate of denudation
and of deposition - On the lapse of time as estimated by years

On the poorness of our palæontological collections - On the
denudation of granitic areas - On the intermittence of geological
formations - On the absence of intermediate varieties in any
one formation On the sudden appearance of groups of species
- On their sudden appearance in the lowest known fossiliferous
strata Antiquity of the habitable earth
.. 345-384

-

CHAPTER X.

ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

On the slow and successive appearance of new species-On their
different rates of change - Species once lost do not reappear -
Groups of species follow the same general rules in their appear-
ance and disappearance as do single species-On Extinction -
On simultaneous changes in the forms of life throughout the
world-On the affinities of extinct species to each other and to
living species- On the state of development of ancient forms-
On the succession of the same types within the same areas —
Summary of preceding and present chapter..
.. 385-421