KE 9781

HARVARD
UNIVERSITYI
LIBRARY
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"But with regard to the material world, we can at least go so far as this-we can perceive that events are brought about not by insulated interpositions of Divine power, exerted in each particular case, but by the establishment of general laws."

WHEWELL: Bridgewater Treatise.

"The only distinct meaning of the word 'natural' is stated, fixed, or settled; since what is natural as much requires and presupposes an intelligent agent to render it so, i. e., to effect it continually or at stated times, as what is supernatural or miraculous does to effect it for once."

BUTLER: Analogy of Revealed Religion.

"To conclude, therefore, let no man out of a weak conceit of sobriety, or an ill-applied moderation, think or maintain, that a man can search too far or be too well studied in the book of God's word, or in the book of God's works; divinity or philosophy; but rather let men endeavour an endless progress or proficience in both."

BACON: Advancement of Learning.

Down, Beckenham, Kent,

First Edition, November 24th, 1859.

Sixth Edition, Jan. 1872.

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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 fol-
lowed, their Effects - Methodical and Unconscious Selection - Un-
known Origin of our Domestic Productions- Circumstances favour-
able to Man's power of Selection

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CHAPTER II.

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VARIATION UNDER NATURE.

5-32

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.

33-47

Its bearing on natural selection-The term used in a wide sense-Geome
trical ratio of increase

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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 rela
tions

48-61

CHAPTER IV.

NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
-its power

Natural Selection - its power compared with man's selection -

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 unfa-
vourable to the results of Natural Selection, namely, intercrossing,
isolation, number of individuals - Slow action Extinction caused by

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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

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Convergence of character-Indefinite multiplication of species-
Summary

Page 62-105

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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

106-132

CHAPTER VI.

DIFFICULTIES OF THE THEORY.

Absence or rarity

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Difficulties of the theory of descent with modification
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

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133-167

CHAPTER VII.

MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL

SELECTION.

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Longevity Modifications not necessarily simultaneous Modifications
apparently of no direct service-Progressive development - Characters
of small functional importance, the most constant-Supposed incom-
petence 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 disbeliev-
ing in great and abrupt modifications
Page 168-204

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CHAPTER VIII.

INSTINCT.

Instincts comparable with habits, but different in their origin-Instincts
graduated Aphides and ants -Instincts variable - Domestic in-
stincts, 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
205-234

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CHAPTER IX.

HYBRIDISM.

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Distinction between the sterility of first crosses and of hybrids — Sterility
various in degree, not universal, affected by close interbreeding, re-
moved 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
tertility Summary

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234-263

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 by 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

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appearance of groups of species On their sudden appearance in

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the lowest known fossiliferous strata - Antiquity of the habitable
earth
Page 264-289

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 develop-
ment of ancient forms - On the succession of the same types within
the same areas Summary of preceding and present chapter 290-315

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CHAPTER XII.

GEOGRAPHICAL DISTRIBUTION.

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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

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CHAPTER XIII.

GEOGRAPHICAL DISTRIBUTION-continued.

316-342

Distribution of fresh-water productions. On the inhabitants of oceanic

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islands - Absence of Batrachians and of terrestrial Mammals - On
the relation of the inhabitants of islands to those of the nearest main-
land - On colonization from the nearest source with subsequent modi-
fication Summary of the last and present chapter

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343-362