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KF 29315

NH 3558.81.24.2 LẦN

V

HARVARD
UNIVERSITY
LIBRARY

FEB 28 1952

Authorized Edition.

ADDITIONS AND CORRECTIONS, TO THE SIXTH EDITION

HISTORICAL SKETCH
INTRODUCTION ..

CONTENTS.

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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
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xiii

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CHAPTER II.

VARIATION Under Nature.

Wide

Variability Individual differences - Doubtful species.
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

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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

Page 75

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

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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-Summary.
.. Page 164

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CHAPTER VI.

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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
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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
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CHAPTER VIII.

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
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

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