Progression, Retrogression and Degression. 73 



without reversion. xA.nd a liost 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 tlie origin 

 of new specific characters on the one hand, and retro- 

 and degressive specific differentiation, which consists in 

 the activation or latency of potentialities already in ex- 

 istence. 



It is obvious that a premutation is necessaiy for pro- 

 gressive but not for retro- and degressive difl:*erentiation. 

 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.^ 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 

 vulgaris peloria (see § 20). 



I have already stated, in Vol. I, Part II, that I regard 

 the mutational period in Oenothera Laniarekiana 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.- We often find in the vegetable kingdom 

 analogous groups of closely related species which are 



^ See Vol. I, Part II and especially § 31, p. 490. 

 ^Vol. I, p. 259. 



