148 THE PLANT 



habitat. This fact distinguishes it from origin by variation, or by mutation. 

 The new form may appear suddenly, often in a single generation, or grad- 

 ually, but in either case it is the result of adaptation that is necessarily 

 advantageous, because it is the result of adjustment to controlling physical 

 factors. Origin by adaptation is perhaps only a special kind of origin by 

 variation, but this might be said with equal truth of mutation. New forms 

 resulting from adaptation are like those produced from mutation, in that 

 they appear suddenly as a rule and without the agency of selection. They 

 are essentially different, inasmuch as their 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 fluctua- 

 tion 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 un- 

 scientific 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 ex- 

 periment. 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. Machaeranthcra cancscens, judging from 

 the numerous minute intergrades betv/een 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. cancscens aspcra, supcrba, rqmosa, ziscosa, etc. Onagra lamarckiana 

 is a mutable species : it comprises many mutants, e. g., Onagra lamarckiana 

 gigas, O. I. nanella, O. I. lata, etc. Galium borcale is an adaptable species: 

 it possesses one distinct ecad, Galium borcale hylocolum, which is the shade 

 form of the species. 



