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VARIATION UNDER DOMESTICATION.
Causes of Variability-Effects of Habit-Correlation of Growth-Inheritance-Char.
acter 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-Principle of Selection anciently followed, its
Effects-Methodical and Unconscious Selection-Unknown Origin of our Domes-
tic Productions-Circumstances favourable to Man's power of Selection, 14
VARIATION UNDER NATURE.
Variability-Individual differences--Doubtful species-Wide ranging, much diffused,
and common species vary most—Species of the larger genera in any country vary
more 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,
. . 46
STRUGGLE FOR EXISTENCE.
Bears on natural selection—The term used in a wide sense-Geometrical powers of
increase-Rapid increase of naturalised animals and plants-Nature of the checks
to increase-Competition universal-Effects of climate-Protection from the num-
ber of individuals-Complex relations of all animals and plants throughout nature
-Struggle for life most severe between individuals and varieties of the same spe-
cies ; often severe between species of the same genus-The relation of organism
to organism the most important of all relations,
. . 60
- CHAPTER IV.
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 samo species-Cir.
cumstances favourable and unfavourable to 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 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, . . . . . . . 1
LAWS OF VARIATION.
Effects of external conditions—Use and disuse, combined with natural selection ;
organs of flight and of vision-Acclimatisation-Correlation of growth-Compen-
sation 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-Re-
versions to long lost characters-Summary,
DIFFICULTIES ON THEORY.
Difficulties on the theory of descent with modification-Transitions-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-Means 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,
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, ostrich, and parasitic bees-Slaye-making ants-Hive-bee,
its cell-making instinct-Difficulties on the theory of the Natural Selection of
instincts-Nouter or sterile insects-Summary, . . . . . . 185
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—Causes of the sterility of first crosses and of
hybrids—Parallelism between the effects of changed conditions of life and cross- ing-Fertility of varieties when crossed and of their mongrel offspring not uni,
versal-Hybrids and mongrels compared independently of their fertility-Sum.
. . . . . . 217
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 vast lapse of time, as inferred
from the rate of deposition and of denudation-On the poorness of our palæonto-
logical collections—On the intermittence of geological formations-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, · · · · · · · · · · · 24
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 gen-
eral rules in their appearance and disappearance as do single species-On Extinc-
tion-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 de-
velopment of ancient forms-On the succession of the same types within the same
areas–Summary of preceding and present chapters, . . . . 273
Present distribution cannot be accounted for by differences in physical conditions-Im-
portance 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 co-extensive
with the world, ·
Distribution of fresh-water productions-On the inhabitants of oceanic islands-Abo
sence of Batrachians and of terrestrial mammals-On the relation of the inhabit.
ants of islands to those of the nearest mainland-On colonisation from the nearest
source with subsequent modification-Summary of the last and present chap-
· · · · ·
YUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: RUDI.
CLASSIFICATION, groups subordinate to groups-Natural system-Rules and difficul.
ties in classification, explained on the theory of descent with modification Classic
fication 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 super-
vening at an early age, and being inherited at a corresponding age-RUDIMENTARY
ORGANS ; their origin explained-Summary, . . . . . 358
RECAPITULATION AND CONCLUSION.
Recapitulation of the difficulties on 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
. . . . . . . . . . 398