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KF 29315

NH 3558.89.24.2.4

V

HARVARD
UNIVERSITY

LIBRARY
FEB 28 1962

Authorized Edition.

CONTENTS.

ADDITIONS AND CORRECTIONS, TO THE Sixth EDITION
HISTORICAL SKETCH
INTRODUCTION

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xiii

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

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 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 relations

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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-

s–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

CHAPTER V.

LAWS OF VARIATION.

Effects of changed conditions—Use and disuse, combined with

natural selection; organs of flight and of vision-Acclimatisa-

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—Species of
the same genus vary in an analogous manner-Reversions to
long-lost characters—Sumınary

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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

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