GENERAL RELATIONS IO5 



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 hydrophytes 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 w^hich 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 



^The Relation of Leaf Structure to Physical Factors. 1905. 



