No. 443] MUTATION L\ PLANTS. 739 



The essential diffcremcs between the two processes appeiir to 

 have been orii^inally set forth by (harks Darwin,' and are treatetl 

 at lenj^h by deVries. The formal distinctions drawn bydeVries 

 appear to need some sli<^ht m<Klification and elaboration in order 

 to make them universally applicable. Thus he holds that con- 

 tinuous, or fluctuating variability occurs only in accordance with 

 Ouetelet's laws, and that it involves only the number, size and 

 weis^ht of organs, and does not include differences in (|ualities. 

 Cultural experiments of various kinds during the last few years 

 have given results in which the qualities as well as the numlK-r, 

 size and structure of organs have been materially altered, but 

 such induced variations or divergences were not transmissible. 

 This jjarticular factor in distinguishing between fluctuating antl 

 mutating variability therefore becomes a safe one, when it is 

 modified to make mutating variability include <Mily newly 

 acquired and transmissible qualities. The |)resence of a plant 

 or an animal parasite may not only change the mechanical fea- 

 tures of an organ but may also cause most radical alterations 

 in its physiological properties. A single example of the latter 

 may be cited in the case of the common species of luiphorbia in 

 which the affected leaves alter their geotropic sensibility in such 

 manner that they change from diageotroi^ism to ai)ogeotropism. 

 Such variations are not transmissible however, and in this lies 

 the true test between mutation and fluctuating variation A 

 still further distinction consists in the fact that mutations ensue 

 in the rudimentary state of the individual, while the alterations 

 in qualities induced by any of the above factors in fluctuating 

 variability may be caused in various stages of the development 

 of the individual, but in a rudimentary stage of the organs 

 concerned. Mutative alterations arise with the individual, are 

 not the direct result of external factors, and are jK-rfectly 

 transmissible, while fluctuating variatit)ns may arise by the influ- 

 ence of external factors at various stages in the individual 

 development, and are not transmissible in their entirety. 



Much of the confusion inevitable to any discussion of the 

 subject may be avoided if it is borne in mind that we h:i'>i"""^ 



lUeVries. .\futationsthcorii, IJd., 2, 1903. 



