Progression, Retrogression and Degression. 73 
without reversion. And a host of new species have 
doubtless arisen on similar lines. 
Coming now to the discussion of the difference be- 
tween our two groups A and B, we draw a distinction 
between progressive specific differentiations or the origin 
of new specific characters on the one hand, and rctro- 
and degressive specific differentiation, which consists in 
the activation or latency of potentialities already in ex- 
istence. 
It is obvious that a premutation is necessary for pro- 
gressive but not for retro- and degressive differentiation. 
For in the case of the former the new potentialities must 
first arise before they can become visible externally, whilst 
in the case of the two latter we are only dealing with 
potentialities already existing. I propose therefore to 
apply the results obtained with Oenothera Laniarckiana 
and the conclusions regarding the premutation period to 
which we arrived, to the further elucidation of this ques- 
tion. 1 It is of course a purely speculative discussion 
that we are embarking on, but one which will, in my 
opinion, materially help in clearing the ground. And I 
may therefore say, in anticipation, that this theory is 
supported by the experiments to be recorded in this sec- 
tion and most strikingly by the history of my Linaria 
z'lilc/aris peloria (see 20). 
I have already stated, in Vol. I, Part II, that I regard 
the mutational period in Oenothera Laniarckiana as a 
type of the mode of origin of species in general; that is 
to say, of the essential form of that process, the pro- 
gressive type. 2 We often find in the vegetable kingdom 
analogous groups of closely related species which are 
1 See Vol. I, Part II and especially 31, p. 490. 
2 Vol. I, p. 259- 
