Authorized Edition. CONTENTS.

ADDITIONS AND CORRECTIONS, TO THE SIXTH EDITION

HISTORICAL SKETCH

Page is

xiii

..

INTRODUCTION

1

CHAPTER I.

VARIATION UNDER DOMESTICATION.

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 followed, their Effects-

Methodical and Unconscious Selection—Unknown Origin of our

Domestic Productions-Circumstances favourable to Man's

power of Selection

7

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 fre-

quently 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

51

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-Com-

petition universal—Effects of climate-Protection from the

number of individuals—Complex relations of all animals and

plants throughout nature—Struggle for life most

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 relations

Page 75

severe

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—Cir-

cumstances 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 Ex-

tinction, 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 ..

97

tion-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—Spec of

the same genus vary in an analogous manner- Reversions to

long-lost characters—Summary

Page 164

CHAPTER VI.

DIFFICULTIES OF THE THEORY.

Difficulties of the theory of descent with modification-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 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

207

CHAPTER VII.

MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL

SELECTION.

Longevity-Modifications not necessarily simultaneous—Modifica-

tions apparently of no direct service—Progressive development

-Characters of small functional importance, the most constant

-Supposed incompetence 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 dis-

believing in great and abrupt modifications

262

CHAPTER VIII.

INSTINCT.

Instincts comparable with habits, but different in their orizin-

Instincts graduated—Aphides and ants—Instincts variable
Domestic instincts, their origin-Natural instincts of the cuckou
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

Page 319