146: THE PLANT 
apply this test critically to every “new species.” Descriptive botany, as. 
practiced at present, will fall into disuse, as scientific standards come to 
prevail, and in its place will appear a real science of taxcnomy. In the latter 
the criteria upon which species are based will be obtained solely by 
experiment. 
184. Fundamental lines of inquiry. There are two primary and 
sharply defined fields of research in experimental evolution, namely, adapta- 
tion in consequence of variation (and mutation), and hybridization. The 
latter constitutes a particular field of inquiry, which is not intimately con-. 
nected with the problems of evolution in nature. In the study. of specific 
adaptation, two questions of profound importance appear. One deals with 
the effects of ancestral fixity or plasticity in determining the amount of 
modification produced by the habitat. These are fundamental problems, 
and a solution of them can not be hoped for until exact and trustworthy data 
have been provided by numerous experimental researches. It thus becomes 
clear that the principal, if not the sole task of experimental evolution for 
years to come is the diligent prosecution of accurate and prolonged experi- 
ment in the modification of plant forms. It seems inevitable that this will 
be carried on along the lines that have already been indicated. Plants will 
be grown in habitats of measured value, or in different intensities of the 
same factor. The relation between stimulus and adjustment will form the 
basis of careful quantitative study, and the final expression of this relation 
in structural modifications will find an exact record in drawings, photo- 
graphs, exsiccati, and biometrical measures. The making of an accurate 
and complete record of the whole course of each experiment of this sort 
is an obligation that rests upon every investigator. Studies in experimental 
evolution will prove time-consuming beyond all other lines of botanical 
research, and the work of one generation should appear in a record so 
perfect that it can be used without doubt or hesitation as a basis for the 
studies of the succeeding generation. 
185. Ancestral form and structure. The significance of the fact that 
some species have been found to remain unaltered structurally under changes 
of habitats that produced striking modifications in others has already been 
commented upon. It is hardly necessary to indicate the important bearing 
which this has upon evolution. The very ability of a plant to undergo 
modification, and hence to give rise to new forms, depends upon the degree 
of fixity of the characters which it has inherited. Stable plants are less 
susceptible of evolution than plastic ones. The latter adapt themselves to 
new habitats with ease, and in each produce a new form, which may serve 
