On the other hand the geographic distribution may be
considered as an indication of the direction of evolution, the
wide-spread forms being regarded as the common parents of the minor
local species.
However, such considerations are only of secondary importance. It must
be borne in mind that an ordinary systematic species may include [47]
many dozens of elementary forms, each of which remains constant and
unchanged in successive generations, even if cultivated in the same
garden and under similar external conditions.
Leaving the violets, we may take the vernal whitlow-grass or _Draba
verna_ for a second illustration. This little annual cruciferous plant
is common in the fields of many parts of the United States, though
originally introduced from Europe. It has small basal rosettes which
develop during summer and winter, and produce numerous leafless
flowering stems early in the spring. It is a native of central Europe
and western Asia, and may be considered as one of the most common
plants, occurring anywhere in immense numbers on sandy soils. Jordan was
the first to point out that it is not the same throughout its entire
range. Although a hasty survey does not reveal differences, they show
themselves on closer inspection. De Bary, Thuret, Rosen and many others
confirmed this result, and repeated the pedigree-cultures of Jordan.
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