DETAILED CONTENTS.
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 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.
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—Competition
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.
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—Circumstances 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 Extinction, 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.
CHAPTER V.
LAWS OF VARIATION.
Effects of changed conditions—Use and disuse, combined with natural
selection; organs of flight and of
vision—Acclimatisation—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.
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.
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.
Longevity—Modifications not necessarily simultaneous—Modifications
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 disbelieving in great and abrupt
modifications.
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 on the theory of the Natural Selection of
instincts—Neuter or sterile insects—Summary.
CHAPTER IX.
HYBRIDISM.
Distinction between the sterility of first crosses and of
hybrids—Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication—Laws governing the sterility of
hybrids—Sterility not a special endowment, but incidental on other
differences, not accumulated by natural selection—Causes of the sterility
of first crosses and of hybrids—Parallelism between the effects of
changed conditions of life and of crossing—Dimorphism and
Trimorphism—Fertility of varieties when crossed and of their mongrel
offspring not universal—Hybrids and mongrels compared independently of
their fertility—Summary.
CHAPTER X.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day—On the nature
of extinct intermediate varieties; on their number—On the lapse of time,
as inferred from the rate of denudation and of deposition—On the lapse of
time as estimated in years—On the poorness of our palæontological
collections—On the intermittence of geological formations—On the
denudation of granitic areas—On the absence of intermediate varieties in
any one formation—On the sudden appearance of groups of species—On
their sudden appearance in the lowest known fossiliferous
strata—Antiquity of the habitable earth.
CHAPTER XI.
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species—On their different
rates of change—Species once lost do not reappear—Groups of species
follow the same general rules in their appearance and disappearance as do
single species—On extinction—On simultaneous changes in the forms
of life throughout the world—On the affinities of extinct species to each
other and to living species—On the state of development of ancient
forms—On the succession of the same types within the same
areas—Summary of preceding and present chapter.
CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical
conditions—Importance of barriers—Affinity of the productions of
the same continent—Centres of creation—Means of dispersal by
changes of climate and of the level of the land, and by occasional
means—Dispersal during the Glacial period—Alternate Glacial periods
in the north and south.
CHAPTER XIII.
GEOGRAPHICAL DISTRIBUTION—continued.
Distribution of fresh-water productions—On the inhabitants of oceanic
islands—Absence of Batrachians and of terrestrial Mammals—On the
relation of the inhabitants of islands to those of the nearest
mainland—On colonisation from the nearest source with subsequent
modification—Summary of the last and present chapter.
CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS:
MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS.
Classification, groups subordinate to groups—Natural system—Rules
and difficulties in classification, explained on the theory of descent with
modification—Classification of varieties—Descent always used in
classification—Analogical or adaptive characters—Affinities,
general, complex and radiating—Extinction separates and defines
groups—Morphology, between members of the same class, between parts of
the same individual—Embryology, laws of, explained by variations not
supervening at an early age, and being inherited at a corresponding
age—Rudimentary Organs; their origin explained—Summary.
CHAPTER XV.
RECAPITULATION AND CONCLUSION.
Recapitulation of the objections to the theory of Natural
Selection—Recapitulation of the general and special circumstances in its
favour—Causes of the general belief in the immutability of
species—How far the theory of Natural Selection may be
extended—Effects of its adoption on the study of Natural
history—Concluding remarks.