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. Jt 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. 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. 1. nanella, O. I. lata, etc. Galium boreale is an adaptable species: 
it possesses one distinct ecad, Galium boreale hylocolum, which is the shade 
form of the species. 
