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Navy

On the generality of intercrosses between individuals of the same 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, ....... 77

CHAPTER V.

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 generio : secondary sexual

characters variable—Species of the same genus vary in an analogous manner—Re-

versions to long lost characters—Summary, ..... 120

CHAPTER VI.

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 difliculty—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, ...... 154

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