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--

Causes of Variability — Effects of Habit and the use or disuse of Parts-

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 Un-

known Origin of our Domestic Productions - Circumstances favour-

able to Man's power of Selection

5-32

-

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 unequally, related to each other,

and in having restricted ranges

33-47

CHAPTER III.

STRUGGLE FOR EXISTENCE.

Its bearing on natural selection The term used in a wide sense- -Geome-

trical ratio of increase

-

-

-

Competition universal-

-

plants Nature of the checks to increase

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 between species of the same genus - The

relation of organism to organism the most important of all rela-

tions

48-61

CHAPTER IV.

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-Circumstances favourable and unfa-

vourable 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

Convergence of character-Indefinite multiplication of species

Summary-

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Page 62-105

CHAPTER V.

LAWS OF VARIATION.

-

Effects of changed conditions. Use and disuse, combined with natural

selection; organs of flight and of vision - Acclimatisation - 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

-

106-132

CHAPTER VI.

DIFFICULTIES OF THE THEORY.

Difficulties of the theory of descent with modification

transition

--

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

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

CHAPTER VII.

MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL

SELECTION.

Longevity - Modifications not necessarily simultaneous Modifications

apparently of no direct service-Progressive development - Characters

of small functional importance, the most constant-Supposed incom-

petence of natural selection to account for the incipient stages of

useful structures-Causes which interfere with the acquisition through

natural selection of useful structures-Gradations of structure with

changed functions-Widely different organs in members of the same

class, developed from one and the same source-Reasons for disbeliev-

ng in great and abrupt modifications

Page 168-204

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CHAPTER VIII.

INSTINCT.

Instincts comparable with habits, but different in their origin-Instincts

graduated Aphides and ants - Instincts variable Domestic in-

stincts, their origin · Natural instincts of the cuckoo, molothrus,

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

205-234

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CHAPTER IX.

HYBRIDISM.

Distinction between the sterility of first crosses and of hybrids - Sterility

various in degree, not universal, affected by close interbreeding, re-

moved 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

universal-Hybrids and mongrels compared independently of their

fertility Summary

234-263

CHAPTER X.

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 intermittence of

geological formations - On the denudation of granitic areas 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

Page 264-289

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CHAPTER XI.

..

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 appearance 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 develop-

ment of ancient forms - On the succession of the same types within

the same areas Summary of preceding and present chapter 290-315

-

CHAPTER XII.

GEOGRAPHICAL DISTRIBUTION.

-

-

Present distribution cannot be accounted for by differences in physical

conditions Importance of barriers Affinity of the productions of

the same continent Centres of creation-Means of dispersal, by

changes of climate and of the level of the land, and by occasional means

--Dispersal during the Glacial period — Alternate Glacial periods in

the north and south

316-342

islands Absence of Batrachians and of terrestrial Mammals On

the relation of the inhabitants of islands to those of the nearest main-

land On colonization from the nearest source with subsequent modi-

-

fication

-

Summary of the last and present chapter

343-362