CONTENTS. 



climate Protection from the number of individuals Complex relations 

 of all animals and plants throughout nature Struggle for 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 Page 44 



CHAPTER IV. 



NATURAL SELECTION ; oa 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 between individuals 

 of the same species Circumstances favourable and unfavourable 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 naturalisation 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 organisation Low forms preserved Convergence of Character 

 Indefinite multiplication of species Summary . . . . . . 57 



CHAPTER V. 



LAWS OF VARIATION. 



Effects of changed conditions Use and disuse, combined with natural selection ; 

 organs of flight and of vision Acclimatisation Correlated variation 

 Compensation and economy of growth False correlations Multiple, 

 rudimentary, and lowly organised 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 van- in an analogous manner Reversions to long-lost characters 

 Summary . . . . . . . . . . . , . . . . 98 



CHAPTER VL 



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 



