3tyi 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 gene- 

 rality 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 naturalization — 

 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 char- 

 acter — Indefinite multiplication of species — Summary. . . 69 



CHAPTER V. 



LAWS OF VARIATION. 



Effects of changed conditions — Use and disuse, combined with 

 natural selection ; organs of flight and of vision — Acclimatiza- 

 tion — Correlated variation — Compensation and economy of 

 growth — False correlations — Multiple, rudimentary, 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 characters — Summary 119 



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 difficulty 



— Natura non facit saltum — Organs of small importance — 

 Organs not in all cases absolutely~perfeet — The law of Unity 

 of Type and of the Conditions of Existence embraced by the 

 theory of Natural Selection 149 



CHAPTER VII. 



MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL 



SELECTION. 



Longevity — Modifications not necessarily simultaneous — Modifi- 

 cations apparently of no direct service — Progressive develop- 



