6 CONTENTS. 



out Nature— Strugiclo for Life most severe hctwccn Individuals and Varieties of 

 the same Species : often severe between Species of the same Genns— The Rela- 

 tion of Organism to Organism the most important of all Relations faoe 69 



CHAPTER IV. 



NATURAL PKLKCTION, OR THE SrUVIVAL OF TIIK FITTEST. 



Natural Selection— its Power compared with Man's Selection — its Power on Char- 

 acters of trifling Importance— its Power at all Ages and on both Sexes — Sexual 

 Selection— On the Generality of Intercrosses bi^tween Individuals of the same 

 Species — Circumstances fiivorable and unfavorable to the Uesults of Natural Se- 

 lection, 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 Nat- 

 ural Selection, through Divergence of Character and Extinction, on the Descend- 

 ants from a Common Parent — Explains the Grouping of all Organic Beings— Ad- 

 vance in Organization— Low Forms preserved — Objections considered— Uniform- 

 ity of certain Characters due to their Unimportance and to their not having been 

 acted on by Natural Selection— Indefinite Multiplication of Species— Summary. 



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 — Com- 

 pensation and Economy of Growtli — False Correlations — Multiple, Rudimentary, 

 and Lowly-organized Structures variable — Parts developed in an Unusual Man- 

 ner are highly variable: Specific Characters more variable than Generic: Second- 

 ary Sexual Characters variable — Species of the same Genus vary in an analogous 

 Manner- Reversions to long-lost Characters — Summary . . . 137 



CHAPTER VI. 



DIFFICULTIES OF THE TIIEORV. 



Difficulties of the Theory of Descent with Modification— Transitions— Absence or 

 Rarity of Transitional Varieties — Transitions in Habits of Life — Diversified Elal)- 

 its in the same Species— Species with Habits widely diO'erent from those of their 

 Allies— Organs of Extreme Perfection — Modes of Traifsition — 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 .... 167 



CHAPTER VII. 



INSTINCT. 



Instincts comjiarable with Ilabils, but different in tlu-ir Origin— Instincts graduated 

 — Aphides and Ants— Instincts variable— Domestic Instincts, their Origin— Nat- 



