On the Origin of Species by Means of Natural Selection: Or, The Preservation ... - Charles Darwin - Google Книги

Книги

Изображения страниц

CONTENTS.

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,

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

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-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,

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-Slave-making ants-Hive-bee,

its cell-making instinct-Difficulties on the theory of the Natural Selection of

instincts-Neuter 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.

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

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,

GEOGRAPHICAL DISTRIBUTION.

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,

GEOGRAPHICAL DISTRIBUTION-Continued.

Distribution of fresh-water productions-On the inhabitants of oceanic islands-Ab-

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-

MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:

MENTARY ORGANS.

EMBRYOLOGY: RUDI-

CLASSIFICATION, groups subordinate to groups-Natural system-Rules and difficul.

ties in classification, explained on the theory of descent with modification-Classi

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,

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

Instruction to Binder.-The Diagram to front page 138.

« Предыдущая Продолжить »