cause may be found at once in the habitat, and since a reversal of stimuli produces, in many cases at least, a reversion in form and structure to the ancestral type. A valid distinction between forms or species upon the basis of constancy is impracticable at the present time. It is doubtful that such a distinction can ever be made in anything like an absolute sense, since all degrees of fluctuation may be observed between constancy and inconstancy. In all events, it is gratuitous to make constancy the essential criterion in the present state of our knowledge. So little is certainly known of it that it is equally unscientific to affirm or to deny its value, and even a tentative statement can not be ventured until a vast amount of evidence has been obtained from experiment. Accordingly, there is absolutely no warrant, other than tradition, for limiting the term species to a constant group. In the evolutionary sense, a species is the aggregate ancestral group and the new forms which have sprung from it by variation, mutation, or adaptation. It should not be regarded as an isolated unit for purposes of descriptive botany; indeed, its use in this connection is purely secondary. It is properly the unit to be used in indicating the primary relationships which are the result of evolution. On the basis of their actual behavior in the production of new forms, species may be distinguished as variable, mutable, or adaptable. The new form which results from variation is a _variant_; the product of mutation is a _mutant_, and that of adaptation, an _ecad_. The following examples serve to illustrate these distinctions. _Machaeranthera canescens_, judging from the numerous minute intergrades between its many forms, is a variable species, i. e., one in which forms are arising by the gradual selection of small variations. It apparently comprises a large number of variants, _M. canescens aspera_, _superba_, _ramosa_, _viscosa_, etc. _Onagra lamarckiana_ is a mutable species: it comprises many mutants, e. g., _Onagra lamarckiana gigas_, _O. l. nanella_, _O. l. lata_, etc. _Galium boreale_ is an adaptable species: it possesses one distinct ecad, _Galium boreale hylocolum_, which is the shade form of the species.