6 Introduction. 



DON, Bateson, Duncker, Johannsen, Macleod, and 

 others, have been active workers in this field. Fluctua- 

 tion is either individual or partial : in the former case 

 we are dealing with the statistical comparison of differ- 

 ent individuals; in the latter with different but homolo- 

 gous organs of the same individual; for example with 

 the leaves of a tree. In both cases the capacity for 

 variation is regarded by those who are competent to 

 judge as a means of adaptation to the environment. 

 Single organs vary partly in mass and weight and partly 

 in number. The former case is referred to by Bateson 

 as continuous variation ; the latter as discontinuous. But 

 these terms are sometimes used by other authors with a 

 different meaning. 



The laws of Mutability are quite different from those 

 of individual variation; but, so far as our scanty infor- 

 mation reaches they are just as independent of the mor- 

 phological nature of the mutating organ. We can dis- 

 tinguish between progressive and retrogressive Mutation. 

 The former results in the origin of a new character ; the 

 latter in the loss of one already existing. It is, obviously, 

 to progressive Mutation, according to this theory, that 

 the main branches of the animal and vegetable genea- 

 logical tree owe their development; but the great major- 

 ity of the cases of the departure of a single species from 

 the type of the systematic group to which it belongs is 

 due to retrogressive Mutation. 



It is to considerations of this kind that the first 

 part of this volume will be devoted. In the first place 

 I shall give a critical revision of the facts on which the 

 theory of Natural Selection of Darwin and Wallace 

 and others is based. In the second I shall deal with 

 some examples of the experimental study of new forms. 



