vi CONTENTS. 



CHAPTEE IV. 



Natural Selection; or the Survival of the Fittest. 



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 unfa- 

 vourable 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 Page 62-105 



CHAPTEE 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 



CHAPTEE VI. 



Difficulties of the Theory. 



Difficulties of 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 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 133-167 



