The question naturally arises how these groups of nearly allied forms may originally have been produced. Granting a common origin for all of them, the changes may have been [51] simultaneous or successive. According to the geographic distribution, the place of common origin must probably be sought in the southern part of central Europe, perhaps even in the vicinity of Lyons. Here we may assume that the old _Draba verna_ has produced a host or a swarm of new types. Thence they must have spread over Europe, but whether in doing so they have remained constant, or whether some or many of them have repeatedly undergone specific mutations, is of course unknown. The main fact is, that such a small species as _Draba verna_ is not at all a uniform type, but comprises over two hundred well distinguished and constant forms. It is readily granted that violets and whitlowgrasses are extreme instances of systematic variability. Such great numbers of elementary species are not often included in single species of the system. But the numbers are of secondary importance, and the fact that systematic species consist, as a rule, of more than one independent and constant subspecies, retains its almost universal validity. In some cases the systematic species are manifest groups, sharply differentiated from one another. In other instances the groups of elementary forms as they are shown by direct observation, have been adjudged by many authors [52] to be too large to constitute species. Hence the polymorphous genera, concerning the systematic subdivisions of which hardly two authors agree. Brambles and roses are widely known instances, but oaks, elms, apples, and pears, _Mentha_, _Prunu_s, _Vitis_, _Lactuca_, _Cucumis_, _Cucurbita_ and numerous others are in the same condition. In some instances the existence of elementary species is so obvious, that they have been described by taxonomists as systematic varieties or even as good species. The primroses afford a widely known example. Linnaeus called them _Primula veris_, and recognized three types as pertaining to this species, but Jacquin and others have elevated these subspecies to the full rank of species. They now bear the names of _Primula elatior_ with larger, _P. officinalis_ with smaller flowers, and _P. acaulis_. In the last named the common flower-stalk is lacking and the flowers of the umbel seem to be borne in the arils of the basal leaves.