EXPERIMENTAL EVOLUTION I49 



187. Methods. The best of all experiments in evolution are those that 

 are constantly being made in nature. Such experiments are readily dis- 

 covered and studied in the case of origin by adaptation ; variants present 

 much greater difficulties, while mutants are very rare under natural condi- 

 tions. 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 plant- 

 house, 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 con- 

 sidered. 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 sud- 

 denness 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 con- 

 stant 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 



