233 
Aspects of Ecology. 
derable group of structural characters, however, with limits not yet 
defined, has clearly a connection with the nature of the habitat, 
as has long been recognised from the comparison of hydrophytes, 
mesophytes and xerophytes. 
The problems as regards the plant are, as with the habitat, 
selection and measurement, but the material is more intricate. 
When the right structural features have been determined upon 
(which in detail often presents difficulties), and rightly measured, 
the investigator is brought face to face with the fundamental 
problem of Ecology, namely, the nature and mechanism of the cor¬ 
relation between habitat and plant, between factors and structure. 
Correlation of Factor, Function and Structure. 
Dr. Clements regards this correlation as close, comparatively 
simple and of wide application, and it is this conviction which 
enables him with unwavering stoutness of heart to plan a complete 
scheme for measuring everything and correlating everything on 
lines which he hopes are final, though the essential nature of the 
correlation has yet to be expounded. 
The connection between the factors of the habitat and the 
structure of the plant is, naturally, Function, and Dr. Clements 
takes the simplest view of the co-ordination of these three, and 
presents such a superficially complete scheme, that we are com¬ 
pelled to try and point out that this simplicity certainly does not 
correspond with the real complexity of the plant as a functional 
machine : the attainment of a complete exposition of these cor¬ 
relations will be practically the last word of philosophical botany. 
Function, says Dr. Clements, is “but the middleman between 
habitat and plant,” and in his anxiety to get everything systematically 
arranged and at once “anticipate the final merging of physiology 
and ecology,” he is prepared to hypothecate a complete set of 
physiological relations of the simplest sort in order to pass, with as 
little resistance as possible, from the characters of the habitat to 
the structural characters of the plant. We may give an outline of 
his conceptions on this subject almost in his own words (pais 
144—151). 
1. “Whatever produces a change in the functions of a plant 
is a stimulus.” 
2. The immediate response to a stimulus is always functional 
(adjustment). The nature and intensity of the stimulus determine 
