1 18 
EVOLUTION 
varieties of the common buttercup (. Ranunculus acris ), one 
of which, var. Steveni Reich., has the stem slightly hairy at 
the base and the segments of the radical leaves not over- 
lapping; while the other, var. vulgaius Jord., has the stem 
densely hairy at the base and the leaf-segments overlapping. 
The student should work through Hooker’s British Flora 
and note the divergences of character usually considered as 
sufficient to mark varieties. He can hardly fail to be struck 
by the apparently trivial nature of the differences, and will 
often be at a loss to understand what value they can possess 
in the struggle for existence. This point cannot be discussed 
here ; we can only refer to what has been said above as to 
the possible value of any variation, and as to correlated 
variation. On the theory of mutation, varieties or “ elemen- 
tary species” may arise at one step, and it is perhaps 
somewhat less difficult to account for varietal characters 
being so often of no apparent value to the plants. 
It is by no means easy to decide when two forms are 
sufficiently separated to be ranked as varieties ; it is essential 
that in several characters their variation should be discon- 
tinuous, yielding a two-humped curve (p. 22), though in 
actual systematic practice this criterion of definite measure- 
ment is seldom employed ; examination of a large number 
of specimens is usually sufficient to enable a judgment to be 
formed as to the continuity or discontinuity of the variation 
of the characters. 
To return to the diagram, imagine the original species A 
now represented by the two varieties a t and k b The latter 
may diverge again into two new varieties, t and u , and 
afterwards a into v and w, and at the stage marked by q the 
species will thus have four varieties. The differences be- 
tween v and w will be slight, most of their characters being 
the same as those of the parent form a ; t and u will be 
rather more widely separated in character. The differences 
between the two pairs, on the other hand, will be consider- 
able, as they are separated by so many generations from the 
common ancestor A. 
The divergence of the four varieties continuing, will 
ultimately become so considerable that the forms must be 
ranked as species. To define exactly what is meant by this 
