F RESEARCHES DURING H. M. S. BEAGLE'S VOYAGE
ROUND THE WORLD.'

_FIFTH THOUSAND._

LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1860.

_The right of Translation is reserved._

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LONDON: PRINTED BY W. CLOWES AND SONS, STAMFORD STREET,
AND CHARING CROSS.

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CONTENTS.

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INTRODUCTION

Page 1

CHAPTER I.

VARIATION UNDER DOMESTICATION.

Causes of Variability--Effects of Habit--Correlation of
Growth--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--Principle of Selection anciently followed, its Effects--Methodical
and Unconscious Selection--Unknown Origin of our Domestic
Productions--Circumstances favourable to Man's power of Selection

7-43

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 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

44-59

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CHAPTER III.

STRUGGLE FOR EXISTENCE.

Its bearing 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 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

60-79

CHAPTER IV.

NATURAL SELECTION.

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 Natural Selection, namely, intercrossing, isolation, number of
individuals--Slow action--Extinction caused by Natural
Selection--Divergence of Character, related to the diversi