x CONTENTS 



PAGE 



CHAPTER 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 be- 

 tween individuals of the same species — Circumstances favorable 

 and unfavorable to the results of Natural Selection, namely, inter- 

 crossing, 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 Char- 

 acter and Extinction, on the descendants from a common parent 

 — Explains the grouping of all organic beings — Advance in organi- 

 zation — Low forms preserved — Convergence of character — Indef- 

 inite multiplication of species — Summary 62 



CHAPTER V 



LAWS OF VARIATION 



Effects of changed conditions — Use and disuse, combined with natural 

 selection; organs of flight and of vision — Acclimatization — Corre- 

 lated variation — Compensation and economy of growth — False 

 correlations — Multiple, rudimentary, and lowly organized struc- 

 tures 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 108 



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 — Di- 

 versified 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 abso- 

 lutely perfect — The law of Unity of Type and of the Conditions 

 of Existence embraced by the theory of Natural Selection . . . 136 



CHAPTER VII 



MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL 

 SELECTION 



Longevity — Modifications not necessarily simultaneous — Modifications 

 apparently of no direct service — Progressive development — Char- 

 acters of small functional importance, the most constant — Sup- 



