CHAPTER II 



THE ORIGIN OF VARIATION 



It is generally held at the present time that there are three 

 main types of variation differing in their mode of origin, viz. : 

 (i) fluctuations or non-heritable somatic variations, (2) the 

 effects of recombination of existing genes, and (3) mutations in 

 the wider sense (Chapter I, p. 4). Most biologists believe 

 that there is a real distinction between spontaneous germinal 

 change, which is heritable, and non-heritable fluctuations, 

 and they experience great difficulty in accepting any evi- 

 dence that changes wrought either on the body cells or on 

 the germ cells by external agencies, by use or by changed 

 habits, are inherited. It is our object in this chapter to examine 

 the evolutionary importance of the different modes of origin 

 of variation. After estimating the importance of those pro- 

 cesses we consider whether fluctuations can ever become 

 hereditarily fixed. We deal with these questions in the 

 following order : 



1. Fluctuations. 



2. The basis of heritable variation. 



3. Recombination. 



4. Mutation in the restricted sense — Gene-mutations. 



5. The inheritance of induced modifications : 



(a) General considerations. 



(b) Experiments. 



(c) Circumstantial evidence. 



(d) Habit-formation. 



(e) Summary. 



Finally, we attempt to summarise the data and to evaluate 

 their importance in the study of evolution. 



Before proceeding with this programme we may consider 

 what importance the origin of variation has in the study of 



