PRECIS OF CONTENTS 



CHAPTER I 



INTRODUCTION 



Use of the term ' variation.' The study of variation in nature as opposed 

 to that of domesticated animals. 



Object of the work. Variation and evolutionary problems. Causes of 

 variation (general). Various other aspects of the problem — mode 

 of occurrence of variation, frequency, limitations, etc. Individual 

 variation, groups and special categories contrasted. Variation and 

 taxonomy. The ' natural population.' 



Formulation of the chief problems involved in the study of natural varia- 

 tions : (i) Causes of variation ; (2) The characteristics of groups and 

 their mode of occurrence in nature ; (3) The origin and causes of 

 isolation ; (4) The causes of correlation ; (5) The spread of new 

 characters ; (6) The relation between variation and the main ten- 

 dencies of evolution. General remarks on the methods of evolutionary 

 study. 



CHAPTER II 



THE ORIGIN OF VARIATION 



The three types of variation : (1) Fluctuations ; (2) Effects of genetic 

 recombination ; (3) Mutations. 



( 1 ) Fluctuations. Difficulty of distinguishing between these and heritable 

 variation by inspection. Plasticity. 



(2) The basis of heritable variation. Mutation and the genetical determina- 

 tion of hereditary characters. Haldane's six modes of genetical 

 representation. 



(3) Recombination. General. Evolutionary value of variation due to this 

 cause. Crossing between species in nature. Recombination princi- 

 pally of value in ' trying-out ' new combinations. 



(4) Gene-mutations. Their origin (general). Difference between produc- 

 tion of new mutations and alteration of mutation-rate. 



Experimental evidence for alteration of mutation-rate. The alleged 

 ' spontaneity ' of mutations. 



(5) ' The inheritance of induced modifications.'' Introductory and historical. 

 Preliminary difficulties discussed. The difficulty of experimental 



