such groups of individuals as prove to be uniform and constant
throughout succeeding generations. The late Alexis Jordan, of Lyons in
France, made extensive cultures in this direction. In doing so, he
discovered that systematic species, as a rule, comprise some lesser
forms, which often cannot easily be distinguished when grown in
different regions, or by comparing dried material. This fact was, of
course, most distasteful to the systematists of his time and even for a
long period afterwards [38] they attempted to discredit it. Milde and
many others have opposed these new ideas with some temporary success.
Only of late has the school of Jordan received due recognition, after
Thuret, de Bary, Rosen and others tested its practices and openly
pronounced for them. Of late Wittrock of Sweden has joined them, making
extensive experimental studies concerning the real units of some of the
larger species of his country.

From the evidence given by these eminent authorities, we may conclude
that systematic species, as they are accepted nowadays, are as a rule
compound groups. Sometimes they consist of two or three, or a few
elementary types, but in other cases they comprise twenty, or fifty, or
even hundreds of constant and well differentiated forms.

The inner constitution of these groups is however, not at all the same
in all cases. This will be seen by the description of some of the more
interesting of them. The European heartsease, from which our
garden-pansies have been chiefly derived, will serve as an example. The
garden-pansies are a hybrid race, won by crossing the _Viola tricolor_
with the large flowered and bright yellow _V. lutea_. They combine, as
everyone knows, in their wide range of [39] varieties, the attributes of
the latter with the peculiarities of the former species.