Distinction Between Species and Varieties. 581 
mental but for the comparative biologist. 1 The elemen- 
tary species are demonstrably the existing units of 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, partly 
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 vespcr- 
tina and L. diurna, 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 distinguishing feature. 
The flowers of the original species must obviously have 
been red, and those of L. vcspcrtina must have become 
white in the same way as those of other white-flowered 
varieties of red species. This view is supported by the 
fact that the colors of the flowers in these two species 
behave in exactly the same way in crosses as they do in 
many varieties, inasmuch as they conform to MENDEL'S 
laws. Other differences between the two campions are 
1 1 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. 
