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 favora
ble to Man's power of Selection

. 31

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.

74

CHAPTER III

STRUGGLE FOR EXISTENCE

Its bearing on natural selection-The term used in a wide sense-Geo-
metrical ratio of increase-Rapid increase of naturalized animals and
plants-Nature of the checks to increase-Competition universal—
Effects of climate-Protection from the number of individuals-Com-
plex 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 organ-
ism to organism the most important of all relations

98

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 be-
tween individuals of the same species-Circumstances favorable and
unfavorable to the results of Natural Selection; namely, intercross-
ing, 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 naturalization-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 organization-Low forms preserved-
Convergence of character-Indefinite multiplication of species-Sum-
mary.

120

CHAPTER V

LAWS OF VARIATION

Effects of changed conditions-Use and disuse, combined with natural
selection; organs of flight and of vision-Acclimatization-Correlated

Variation-Compensation and economy of growth-False correlations
-Multiple, rudimentary, and lowly organized 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

. 190

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

CHAPTER VII

233

MISCELLANEOUS OBJECTIONS TO THE THEORY OF
NATURAL SELECTION

Longevity-Modifications not necessarily simultaneous--Modifications ap-
parently 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 struc-
tures-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, de-
veloped from one and the same source-Reasons for disbelieving in
great and abrupt modifications

288

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-Diffi-
culties of the theory of the Natural Selection of instincts-Neuter or
sterile insects—Summary

346

ADDITIONS AND CORRECTIONS

TO THE SIXTH EDITION

Numerous small corrections have been made in the last and present editions on various subjects, according as the evidence has become somewhat stronger or weaker. The more important corrections and some additions in the present volume are tabulated on the following pages, for the convenience of those interested in the subject, and who possess the fifth edition. The second edition. was little more than a reprint of the first. The third edition was largely corrected and added to, and the fourth and fifth still more largely. As copies of the present work will be sent abroad, it may be of use if I specify the state of the foreign editions. The third French and second German editions were from the third English, with some few of the additions given in the fourth edition. A new fourth French edition has been translated by Colonel Moulinié; of which the first half is from the fifth English, and the latter half from the present edition. A third German edition, under the superintendence of Professor Victor Carus, was from the fourth English edition; a fifth is now preparing by the same author from the present volume. The second American. edition was from the English second, with a few of the