Distinction Between Species and Varieties. 581 



mental but for the comparative biologist.^ The elemen- 

 tary species are demonstrably the existing units ul the 

 system; whilst the larger species are only aggregations 

 of these. They will therefore be discussed in dealing 

 with the question of the practical differences between spe- 

 cies and varieties. 



But, before I proceed to this, reference must be made 

 to the more complicated but more common case in which 

 two closely related forms differ from one another, i)artly 

 by progressive and partly by retrogressive or degressive 

 characters. To judge by the former they should be re- 

 garded as elementary species, by the latter however, as 

 derivative varieties; and as they are hardly allowed to be 

 in our system both at the same time, we must make a 

 decision one way or the other 



With a view to clearing up these difficulties let me 



deal with a particular instance, and select Lychnis vcspcr- 



tina and L. diiirna, which are regarded by several sys- 



tematists as belonging to one species, Lychnis dioica. If 



we regard these two forms as having been derived from 



a common original ancestor, and consider their individual 



characters, the difference in the color of the flowers 



stands out as the most striking distmguishing feature. 



The flowers of the original species must obviously have 



been red, and those of L. vespertina must have become 



white in the same way as those of other white-flnwercd 



varieties of red species. This view is suj^ported by the 



fact that the colors of the flowers in these two species 



behave in exactlv the same way in crosses as they do in 



many varieties, inasmuch as they conform to Mexdki/s 



laws. Other differences between the two campic^ns are 



^ T do not propose to enter here into the question of the desirabil- 

 ity of a ternary nomenclature (see p. 65) ; it is entirely a question of 

 convention. 



