602 Validity of the Doctrine of Mutation. 
to some other one ; and especially amongst American in- 
vestigators the tendency has been in recent years to pro- 
ceed as far as possible in this direction. 
If we look for a fixed point among these oscillating 
opinions we may well choose the view repeatedly ex- 
pressed by DARWIN himself, that it is possible to imagine 
that characters may originate by a slow process, but may 
disappear all of a sudden. 1 In combining this with the 
distinction made in the first part of this volume between 
progressive, retrogressive and degressive formation of 
species, the proposition would run: Progressive forma- 
tion of species may occur slowly and gradually, whilst 
retrogressive and degressive specific differentiation is due 
to mutations. Progressive differentiation consists in the 
formation of a new character which was not previously 
present; whilst retrogressive and degressive differentia- 
tion consists in the transference of internal factors, al- 
ready present, from one condition to another. In the for- 
mer case the active unit becomes latent ; in the latter the 
latent becomes active, or the semi-latent semi-active ; but 
the material vehicles of these characters remain funda- 
mentally the same throughout ; nothing new arises in the 
idioplasma. 2 
In horticulture, as we have seen, mutations are largely 
of the retrogressive or degressive kind. Discontinuous 
formation of species on the progressive line is much 
rarer. Nevertheless I believe that my researches with 
Ocnoihera have contributed instances which may demon- 
strate the occurrence of progressive mutations in this 
species at any rate. Obviously there is a great need of 
1 With reference to this point see the valuable critique by L. 
PLATE, Ucbcr Bedcutung und Tragweite des Darwin' schen Selections- 
t>rinci[>s, 1900, p. 37 and elsewhere. 
~ See below, 9-11. 
