De Vries points out some of the inconsistencies of those who have attempted to discriminate between varieties and species. The only rule that can be adhered to is that a variety differs from a species to which it belongs in only one or in a few characters. Most so-called varieties in nature are really elementary species, which differ from their nearest relatives, not in one character only, but in nearly all their characters. There is no ground, De Vries states, for believing them to be varieties. If it is found inconvenient to rank them under the names of the old Linnæan species, it will be better, perhaps, to treat them as subspecies, but De Vries prefers to call them elementary species. In regard to the distribution of species in nature, it may be generally stated that the larger the geographical domain so much the larger is the number of elementary species. They are found to be heaped up in the centre of their area of distribution, but are more scattered at the periphery. In any one locality each Linnæan species has as a rule only one or a few elementary species. The larger the area the more numerous the forms. From France alone Jordan had brought together in his garden 50 elementary species of _Draba verna_. From England, Italy, and Austria there could be added 150 more. This polymorphism is, De Vries thinks, a general phenomenon, although the number of forms is seldom so great as in this case. Amongst animals this great variety of forms is not often met with, yet amongst the mammalia and birds of North America there are many cases of local forms or races, some of which at least are probably mutations. This can only be proven, however, by actually transferring the forms to new localities in order to find out if they retain their original characters, or become changed into another form. It seems not improbable that many of the forms are not the outcome of the external conditions under which the animal now lives, but would perpetuate themselves in a new environment.