142 Retrograde Varieties 



Heretofore we have generally assumed, that 

 varieties differ from the parent species in a sin- 

 gle character only, or at least that only one need 

 be considered. We now come to the study of 

 those varieties, which differ in more than one 

 character. Of these there are two types. In 

 the first the points of dissimilarity are inti- 

 mately connected with one another, in the 

 second they are more or less independent. 



The mutually related peculiarities may be 

 termed correlative, and we therefore speak, in 

 such cases, of correlative variability. This 

 phenomenon is of the highest importance and 

 is of general occurrence. But before describing 

 some examples, it is as well to note that in the 

 lecture on fluctuating variability, cases of a 

 totally different nature will be dealt with, which 

 unfortunately are designated by the same 

 term. Such merely fluctuating variations are 

 therefore to be left out of the present dis- 

 cussion. 



The purple thorn-apple, which is considered 

 by some writers as a variety of the white-flow- 

 ered species or Datura Stramonium, and by 

 others as a separate species, D. Tatida, will 

 serve as an illustration. But as its distinguish- 

 ing attributes, as far as we are concerned with 

 them here, are of the nature described above as 

 characteristic of varietal peculiarities no ob- 



