xviii CONTENTS. 



Pass. 

 life most severe between individuals and varieties of tlie 

 same species: often severe between species of the same 

 genus — The relation of organism to organism the most 

 important of all relations 57 



CHAPTER IV. 



NATURAJj SELECTION; OR THE SUBVIVAL OP 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 favorable and unfavorable 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 naturaliza- 

 tion — 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 organization — Low forms preserved — 

 Convergence of character — Indefinite multiplication of 

 species — Summary 73 



CHAPTER V. 



LAWS OF VARIATION. 



Effects of changed conditions — Use and disuse, combined 

 with natural selection; organs of flight and of vision — 

 Acclimatization — Correlated variation — Compensation and 

 economy of growth — False correlations — Multiple, rudi- 

 mentary and lowly organized structures variable — Parts 

 developed in an unusual manner ai'e highly variable; 

 specific characters more variable than generic; secondaiy 

 sexual characters variable — Species of the same genus vary 

 in an analogous manner — Reversions to long-lost charac- 

 ters — Summary 126 



CHAPTER 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 diflBculty — Natura non facit saltum — Organs of 



