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Aspects of Ecology. 
through which the stimuli work to the response should be so con¬ 
structed as to determine this. Whether this is so or not is a 
matter for precise investigation in any given case ; even if propor¬ 
tionately hold in both, it is desirable to keep these two categories 
of causation apart. 
We turn now to consider the second and fourth principles, 
which determine the relation of “adjustment” and “adaptation.” 
If external causes affecting function were truly stimuli, 
the term “adjustment” as suggesting something akin to adaptation 
would be suitable, but not otherwise. 
We find no clear statement of the intended relation between 
adjustment and adaptation as parts of one chain of causation, and 
so we have tried to think out what our author understands by 
change of function being followed by a corresponding change of 
structure. Let us consider the effect of bringing a plant into 
increased dryness of air. Suppose the drier air increases the 
evaporation, i.e. transpiration, 10% per unit area, a change of 
structure corresponding to this would be such an enlarging of 
stomata or thinning of cuticle as would have effected a 10% increase 
in the previous conditions. If any adaptation followed, it would 
be an antagonistic change of structure to bring the total transpi¬ 
ration back to the original suitable amount, say by reducing the 
leaf area by 10% or by thickening the leaf, etc. So it is with roots in 
general as with the transpiring organs ; an alteration of water- 
content of air or of soil produces antagonistic structural change, 
tending to neutralise the effect of the change. This tendency to 
keep to a definite amount of transpiration must in some cases be 
correlated to questions of quantity of growth, for the chief 
biological significance of transpiration is in providing mineral 
elements for further formation of new matter. The amount of 
growth a plant performs is partly an individual matter and partly 
a question of temperature, so that we traverse a complex nexus of 
causation, and we might well get the following state of things—-that 
in regions of low temperature, keeping down the amount of growth, 
a plant would respond to drier air by structural change antagonistic 
to increased transpiration, while in a warmer region it might 
respond to drier air by greater development of root-system and so 
obtain more salts for the greater possibility of growth. Other such 
schemes of correlation could be suggested for future experiment. 
The quantity of growth characteristic of a given form is an impor¬ 
tant qualitative structural character, and one to which no attention 
seems to be paid in this work. 
