=184. Fundamental lines of inquiry.= There are two primary and sharply defined fields of research in experimental evolution, namely, adaptation in consequence of variation (and mutation), and hybridization. The latter constitutes a particular field of inquiry, which is not intimately connected 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 experiment 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, photographs, 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 as the starting point of a phylum. There is at present no clue whatever as to what calls forth this essential difference in behavior. This is not surprising in view of the fact that there have been no comparative experimental studies of stable and plastic species. Until these have been made, it is impossible to do more than to formulate a working hypothesis as to the effect of stability, and an explanation of the forces which cause or control it is altogether out of the question.