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Causes of Variability-Effects of Habit and the use or disuse 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
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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 fre-
quently 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
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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
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
207
CHAPTER VII.
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL
SELECTION.
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-Reasons 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