22 THE PHYLOGENETIC METHOD IN TAXONOMY. 



deal primarily with the transfer within the habitats of the same climax, i. e., between 

 the climax and one or more serai habitats, or the latter alone. Such transplants are 

 especially advantageous, as they are made with the minimum of labor, and usually give 

 the most striking results in the form of ecads. Planting seedlings from the greenhouse 

 or nursery, or sowing seeds under proper precautions, takes advantage of the fact that 

 the seedling is usually more plastic than the adult, but it is subject to many more 

 dangers during ecesis. Consequently, while they are standard methods in experi- 

 mental vegetation, they are little used as yet in experimental evolution. The 

 modification of the habitat is one of the best of methods, chiefly because it permits 

 the modification of a whole group of individuals in position. It is further valuable in 

 furnishing a check upon the behavior of related individuals transplanted to conditions 

 similar to those produced by the modification. Its one disadvantage lies in the labor 

 sometimes involved in bringing about an effective change of conditions, especially in 

 forests. In such cases, however, Nature frequently steps in and brings about the 

 desired result by the fall of a tree, by fire, or wind-throw. In woodland, scrub, grassland, 

 or herbaceous communities it is often an easy task to change efifectively the light inten- 

 sity, water-content, air-content, etc. 



In the case of reciprocal transplants, it is fairly certain that conditions will not be too 

 extreme for either plant, but with climatic and edaphic transplants there is no definite 

 assurance, at least in the beginning. To obviate this, a transplant sequence is used, 

 by which plants are transferred to one or more intermediate zones or habitats. If 

 conditions are too extreme in the last situation of the series and the plants are lost, those 

 of the next less extreme habitat will serve to show the limits of adaptation. When 

 plants are not plastic, however,extreme conditions furnish the only method of breaking 

 the structural habit and thus permitting adaptation (cf. Turesson, 1922). 



As already indicated, a certain amount of control can be exerted over physical factors 

 in the case of field experiments. With respect to the primary factor, this is secured 

 when the water-content of an area is increased or diminished, or the light intensity 

 changed by clearing or shading, especially when part of the group or community is left 

 under the original conditions to serve as a check. In a sense, moreover, a kind of control 

 is assured when plants are transferred from sun to shade or from wet to dry. Garden 

 experiments resemble those in the field in the extent to which factors can be controlled 

 or manipulated, and in both complete measurements of all the major factors are indis- 

 pensable. In the greenhouse, the opportunities for control and manipulation are much 

 greater, and studies of adaptation to definite amounts of factor stimuli can be carried 

 on with much greater convenience and certainty, especially where a sequence of intensities 

 is desired. The greenhouse makes it possible to equalize in large degree all the factors 

 except the one to be studied, and thus permits the more exact causal analysis of results 

 obtained in the field. For this, measurement of the factors is as indispensable as in 

 the field, since this alone permits the correlation of definite quantities of response with 

 equally definite amounts of the controlling factor. 



Objectives. — The first great object of the experimental method is to determine the 

 relationship by descent of the species and variads already in existence. In doing this 

 it necessarily deals with the production of new variads under known conditions, and 

 this leads to the study of the whole question of the causes and methods by which new 

 forms arise. The latter opens up the fundamental problems of the origin and trans- 

 mission of new characters, the solution of which is possible only through the widest 

 range of experiments under measured conditions. In close relation to the origin of forms 

 stands the experimental study of the criteria employed to distinguish variads, species, 

 and genera. This is of particular value to existing taxonomy, and hence is one of the 

 first as well as one of the easiest points of attack. In this the correlation that exists 



