EXPERIMENTAL EVOLUTION ERI 
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 wil! preclude origin by adaptation to 
a large degree, on account of the need for profound readjustment. The 
general rule followed by most polydemics 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 production 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 iactors 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 unusual attention, since pollination, iso- 
lation, etc., are often decisive factors in origin by variation and in ie per- 
sistence of mutants. 
Measurements of adjustment, i. e., functional response to the direct 
factor concerned, are extremely valuable, but not altogether indispensable 
