xviii CONTENTS. 



Pagb. 

 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 57 



CHAPTER IV. 



NATURAL SELECTION; OB THE SUBVIVAL 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 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 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 charac- 

 ters — Summary ]^26 



CHAPTER Vr. 



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 



