=187. Methods.= The best of all experiments in evolution are those that are constantly being made in nature. Such experiments are readily discovered and studied in the case of origin by adaptation; variants present much greater difficulties, while mutants are very rare under natural conditions. The method which makes use of these experiments may be termed the _method of natural experiment_. The number of ecads which appear naturally in vegetation is limited, however, and it is consequently very desirable to produce them artificially, by the _method of habitat culture_. This method, while involving more labor than the preceding, yields results that are equally conclusive, and permits the study of practically every species. The _method of control culture_, which is carried on in the planthouse, naturally does not possess the fundamental value of the field methods. It is an invaluable aid to the latter, however, since it permits the physical factors to be readily modified and controlled. All these methods are based on the indispensable use of instruments for the measurement of physical factors. METHOD OF NATURAL EXPERIMENT =188. Selection of species.= Species that are producing variants or ecads are found everywhere in nature; those which give rise to mutants seem, however, to be extremely rare. Consequently, mutants can not be counted upon for experimental work, and their study scarcely needs to be considered. When a mutant is discovered by some fortunate chance, the mutable species from which it has sprung, and related species as well, should be subjected to the most critical surveillance, in the hope that new mutants will occur or the original one reappear. On account of the suddenness with which they appear, mutants do not lend themselves readily to natural experiment, and after they have once been discovered, inquiry into the causes and course of mutation is practicable only by means of habitat and control cultures. Among variable species, those are most promising that show a wide range of variation and are found in abundance over extensive areas. A species which occurs in widely separated, or more or less isolated areas, furnishes especially favorable material for investigation, since distance or physical barriers partly eliminate the leveling due to constant cross-fertilization. The individuals or groups which show appreciable departure from the type are marked and observed critically from year to year. The direction of the variation and the rapidity with which small changes are accumulated can best be determined by biometrical methods. Representative individuals of the species and each of its variants should likewise be selected from year to year. After being photographed, these are preserved as exsiccati, and with the photographs constitute a complete graphic record of the course of variation. When the latter is made evident in structural feature also, histological slides are an invaluable part of the record.