Progressive, Retrogressive, Degressive Mutations. 569 
tions; and it does not by any means seem superfluous to 
ask ourselves from time to time what has already 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 
