EXPERIMENTAL EVOLUTION I5I 



areas that do not show efficient differences of water or light will contain no 

 ecads of their common species, and also that extreme differences in the 

 amount of either of these two factors will preclude origin by adaptation to 

 a large degree, on account of the need for profound readjustment. The 

 general rule followed by most poly demies is that sun species will give rise 

 to shade forms, and vice versa, and that xerophytes will produce forms of 

 hydrophytic tendency, or the converse, when the areas concerned are not too 

 remote, and the water or light differences are efficient, but not inhibitive. 

 Some species are capable of developing naturally two series of ecads, one 

 in response to light, the other to water-content, but they, unfortunately, have 

 been found to be rare. Greatly diversified regions, such as the Rocky 

 mountains, in which alternation is a peculiarly striking feature of the vegeta- 

 tion, are especially favorable to the pro<luction of ecads, and hence for the 

 study of natural experiments in origin by adaptation. 



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 direc- 

 tion 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 meas- 

 urement 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 imusual attention, since pollination, iso- 

 lation, etc., are often decisive factors in origin by variation and in Uie per- 

 sistence of mutants. 



Measurements of adjustment, i. e., functional response to the direct 

 factor concerned, are extremely valuable, but not altogether indispensable 



