GENERAL RELATIONS 105 
the same stimuli. It seems probable that plants which have reacted to sun- 
light for hundreds of years will respond less readily to shade than those 
which have grown in the sun for a much shorter period. This hypothesis 
is not susceptible of proof in nature because it is ordinarily impossible to 
distinguish species upon the basis of the time .during which they have 
occupied one habitat. Evidence and ultimate proof, perhaps, can be obtained 
only by field and control experiments, in which the time of occupation of 
any habitat is definitely known. Even in this case, however, it is clear 
that antecedent habitats will have left effects which can neither be traced nor 
ignored. Additional support is given this view by the fact that extreme) 
types, both ecological and taxonomic, are the most stable. Intense xero- 
phytes and hydrophytés are much more fixed than mesophytes, though the 
intensity of the stimulus has doubtless as great an influence as its duration. 
Composites, labiates, grasses, orchids, etc., are less plastic than ranals, 
rosals, etc., but there are many exceptions to the apparent rule that fixity 
increases with taxonomic complexity. At present it seems quite impossible 
to suggest an explanation of the rule. Recent experiments indicate that 
there may be ancestral fixity of function, as well as of structure. It has 
been found, for example, that the flowers of certain species always react 
normally to the stimuli which produce opening and closing, while others 
make extremely erratic response. If further work confirms this result and 
extends it to other functions, the necessity of arriving at a better under- 
standing of fixity will be greatly emphasized. 
It is impossible to make progress in the study of adaptation without 
recognizing the fundamental importance of ancestral fixity as a factor. 
E. S. Clements! has shown that a number of species undergo pronounced 
changes in habitat without showing appreciable modification. Consequently, 
it is incorrect to assume that each habitat puts a structural impress upon 
every plant that enters it. For this reason, the writer feels that the current 
explanation of xerophytic bog plants, etc., is probably wrong, and that the 
discrepancy between the nature of the habitat and the structure of the plant 
is to be explained by the persistence of a fixed ancestral type. The anomaly 
is scarcely greater than in cases that have proved capable of being 
explained. 
150. The law of extremes. When a stimulus approaches either the 
maximum or minimum of the factor for the species concerned, response 
becomes abnormal. The resulting modifications approach each other and 
in some respects at least become similar. Such effects are found chiefly in 
1The Relation of Leaf Structure to Physical Factors. 1905. 
