76 Variation 



and in horticulture it gives rise to numerous varieties, which have in 

 the past been preserved, either on account of their usefulness or 

 beauty, or simply as fancy-types. In fact the possession of numbers of 

 varieties may be considered as the main character of domesticated 

 animals and cultivated plants. 



In the case of retrogressive and degressive mutability the internal 

 cause is at once apparent, for it is this which causes the disappear- 

 ance or reappearance of some character. With progressive mutations 

 the case is not so simple, since the new character must first be pro- 

 duced and then displayed. These two processes are theoretically 

 different, but they may occur together or after long intervals. 

 The production of the new character I call premutation, and the 

 displaying mutation. Both of course must have their external as 

 well as their internal causes, as I have repeatedly pointed out in my 

 work on the Mutation Theory 1 . 



It is probable that nutrition plays as important a part among the 

 external causes of mutability as it does among those of fluctuating 

 variability. Observations in support of this view, however, are too 

 scanty to allow of a definite judgment. Darwin assumed an accumu- 

 lative influence of external causes in the case of the production of new 

 varieties or species. The accumulation might be limited to the 

 life-time of a single individual, or embrace that of two or more 

 generations. In the end a degree of instability in the equilibrium of 

 one or more characters might be attained, great enough for a character 

 to give way under a small shock produced by changed conditions of 

 life. The character would then be thrown over from the old state 

 of equilibrium into a new one. 



Characters which happen to be in this state of unstable equi- 

 librium are called mutable. They may be either latent or active, 

 being in the former case derived from old active ones or produced as 

 new ones (by the process, designated premutation). They may be 

 inherited in this mutable condition during a long series of genera- 

 tions. I have shown that in the case of the evening primrose of 

 Lamarck this state of mutability must have existed for at least 

 half a century, for this species was introduced from Texas into 

 England about the year 1860, and since then all the strains derived 

 from its first distribution over the several countries of Europe show 

 the same phenomena in producing new forms. The production of 

 the dwarf evening primrose, or Oenothera nanella, is assumed to be 

 due to one of the factors, which determines the tall stature of the 

 parent form, becoming latent ; this would, therefore, afford an example 

 of retrogressive mutation. Most of the other types of my new 

 mutants, on the other hand, seem to be due to progressive mutability. 



1 Die Mutationstheorie, 2 vols., Leipzig, 1901. 



