x PRfiCIS OF CONTENTS 



proof — question of the stock used in experiment, elimination of 

 selection, etc. 



(a) General. 



(b) Experiments. 



(c) Circumstantial evidence. 



(d) Habit-formation. 



(e) Summary. 



(6) General Conclusions. 



CHAPTER III 



THE CATEGORIES OF VARIANT INDIVIDUALS 



Historical. Various types of categories and the terms used to designate 

 them. The individual, (i) Taxonomic categories. Use of the 

 various terms. Taxonomy and population-analysis. (2) Palaeonto- 

 logical categories. Lineages and bioseries. Palaeontology and neon- 

 tology. (3) Geographical categories. Terms used to designate 

 various kinds of groups. Concept of the ' Rassenkreis.' (4) Genetical 

 and reproductive categories. Various terms employed. The recon- 

 ciliation of genetical and taxonomic categories. (5) Physiological 

 categories. General conclusions. 



CHAPTER IV 



THE DISTRIBUTION OF VARIANTS IN NATURE 



Preliminary considerations. Methods of distinguishing heritable variation 

 from fluctuations apart from experiment. Intermediacy. Variation 

 and size of area. The chief modes of occurrence of variants. 



( 1 ) Individual variation, principal modes of occurrence and examples. 



(2) Polymorphism. Discussion of the term. Examples of the phenomenon 

 in land snails, Lepidoptera, etc. 



(3) Geographical variation. Introduction and general discussion as to 

 whether geographical variation is characteristic of some groups and 

 not of others. Rensch's views. Conclusions on this subject. Examples 

 of geographical variation. 



(4) Physiological races. Degrees of differentiation. General summary. 



CHAPTER V 



ISOLATION 



Two main kinds of isolation — geographical and topographical separation 

 and the prevention of sexual intercourse. General discussion on their 

 effects and interaction. Correlation of the degree of isolation with 

 that of divergence. Time necessary for the establishment of new 

 species. 



Topographical isolation. General. Capriciousness of endemism on islands. 

 Difficulty of the problem. Peculiar characteristics of endemic species. 

 Relation between numerical abundance and rate of evolution. 



