Introduction

1. Variation under Domestication

• 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 aand 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

• 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

• Its bearing on natural selection
• The term used in a wide sense
• Geometrical ratio 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 plants and animals 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

• 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 the results of natural selection, namely, intercrossing, 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
• Advance in organisation
• Low forms preserved
• Convergence of character
• Indefinite multiplication of species
• Summary

• 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

• Difficulties to 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 inportance
• 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

• Longevity
• Modifications not necessarily simultaneous
• Modifications 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
• Gradationsof structure with changed functions
• Widely different organs in members of the same class, developed from one and the same source
• Reasons for disbelieving in great and abrupt modifications

Index

• Mutual Aid Amongst Birds