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
palaeontological 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.
GLOSSARY OF SCIENTIFIC TERMS.
INDEX.

ORIGIN OF SPECIES.

INTRODUCTION.

When on board H.M.S. Beagle, as naturalist, I was much struck with certain facts in the distribution of the organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of that continent. These facts, as will be seen in the latter chapters of this volume, seemed to throw some light on the origin of species—that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it