Page images


Additions And Corrections, To The Sixth Edition .. Page ix

Historical Sketch #_ x;j;

Introduction ^


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 <j


Variation Under Nature.

.+, Variability — Individual differences — Doubtful species Wide

ianging, much diffused, and common species, vary most

►H Species of the larger genera in each country vary more fre-

Q quently than the species of the smaller genera—Many of the
species of the larger genera resemble varieties in being very

ZO closely, but unequally, related to each other, and in havin?

CO restricted ranges 5j


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


Natural Selection; Ob The Survival or 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


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


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


Miscellaneous Objections To The Theory Of Natural

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—Keasons for dis-
believing in great and abrupt modifications 262


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


The Diagram to front page 140, VoL L, and to face the latter part
of the Volume.



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 page, 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 Moulinie; 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

« PreviousContinue »