Progressive, Retrogressive, Degressive Mutations. 569 



tions; and it does not by any means seem snpeiiluous to 

 ask ourselves from time to time what has ah-eady been 

 achieved and what is to be done in the immediate future. 

 In doing so I shall have to keep a close rein on myself 

 and, whenever possible, conform to the opinions of rec- 

 ognized authorities, limiting my own views to such points 

 as may throw light on the relation between those opinions 

 and the theory of mutation. I shall try to avoid ques- 

 tions of minor importance or concerning uncertain or 

 subordinate points; the literature of the subject has long 

 since grown so prodigiously that it is no longer possible 

 to keep pace with it. 



My only object is to demonstrate the applicability of 

 the theory of mutation to the main conclusions of the 

 doctrine of evolution ; and only to do this briefly and 

 sometimes no more than in outline. New theories and 

 new hypotheses I shall not have to introduce, the more 

 so, as I am convinced that the doctrine of mutation will 

 lead everywhere to a simplification and a clearer concep- 

 tion of the problems. The prospect of recognition of a 

 theory rests on the one hand on its empirical foundation, 

 and on the other on its suggestiveness and the number of 

 facts which it explains. Therefore the consideration of 

 this latter point will now be my task. I shall devote the 

 several chapters of this part to the various problems 

 involved. 



§ 2. PROGRESSIVE, RETROGRESSIVE AND DEGRESSIVE 



MUTATIONS. 



Progress in organic nature consists essentially in in- 

 crease of differentiation. The peculiarities which go to 

 make up the individual character of the species become 



