=189. Determination of factors.= For the critical investigation of the origin of new forms, an exact knowledge of the factors of the habitat, both physical and biotic, is imperative. In the case of variable species, these factors determine what variations are of advantage, and thereby the direction in which the species can develop. They are the agents of selection. With mutants, the factors of the habitat are apparently neither causative nor selective, though it seems probable that further study of mutants will show an essential connection between mutant and factor. In any event, the persistence of a mutant in nature, and its corresponding ability to initiate new lines of development, is as much dependent upon the selection exerted by physical and biotic factors as is the origin of variants. Physical factors are causative agents in the production of ecads, as has been shown at length elsewhere. The form and structure of the ecad are the ultimate responses to the stimuli of light or water-content, and the quantitative determination of the latter is accordingly of the most fundamental importance. The measurement of factors has been treated so fully in the preceding chapters that it is only necessary to point out that the thorough investigation of habitats by instruments is as indispensable for the study of experimental evolution as for that of the development and structure of the formation. Furthermore, it is evident that a knowledge of physical factors is as imperative for habitat and control cultures as for the method of natural experiment. In the latter, however, the biotic factors demand unusual attention, since pollination, isolation, etc., are often decisive factors in origin by variation and in the persistence of mutants. Measurements of adjustment, i. e., functional response to the direct factor concerned, are extremely valuable, but not altogether indispensable to research in experimental evolution. This is due to the fact that a knowledge of adjustment is important in tracing the origin of new forms only when adjustment is followed by adaptation, and in all such cases the ratio between the two processes seems to be more or less constant. In the present rudimentary development of the subject, however, it is very desirable to make use of all methods of measuring functional responses to water and light that are practicable in the field. Certain methods that are difficult of application in nature may be used to advantage in control cultures, and the results thus secured can be used to interpret those obtained from natural experiments and field cultures.