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 taxonomy. 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 



