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[63]
LECTURE III
ELEMENTARY SPECIES OF CULTIVATED PLANTS
Recalling the results of the last lecture, we see that the species of
the systematists are not in reality units, though in the ordinary course
of floristic studies they may, as a rule, seem to be so. In some cases
representatives of the same species from different countries or regions,
when compared with one another do not exactly agree. Many species of
ferns afford instances of this rule, and Lindley and other great
systematists have frequently been puzzled by the wide range of
differences between the individuals of a single species.
In other cases the differing forms are observed to grow near each other,
sometimes in neighboring provinces, sometimes in the same locality,
growing and flowering in mixtures of two or three or even more
elementary types. The violets exhibit widespread ancient types, from
which the local species may be taken to have arisen. The common
ancestors of the Whitlow-grasses are probably not to be found [64] among
existing forms, but numerous types are crowded together in the southern
part of central Europe and more thinly scattered elsewhere, even as far
as western Asia. There can be little doubt that their common origin is
to be sought in the center of their geographic distribution.
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