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Aspects of Ecology . 
whether this is followed by a corresponding change in structure 
(adaptation). Many responses are functional alone. 
3. The amount of response is proportional to the stimulus for 
a given individual. 
4. “Axiomatically there is ordinarily an essential corres¬ 
pondence between the amount of adaptation and that of 
adj ustment.” 
Nothing could be simpler than this which is the conception for 
an individual plant. If different species are to be compared we 
find the following additional conception. 
5. “Many species are extremely plastic and respond to slight 
stimuli, others are comparatively fixed and respond only to much 
greater stimuli. For different plants response is only equal when 
they are equally plastic. This correspondence between adjustment 
and adaptation is profoundly affected by the structural fixity of the 
plant.” 
We do not find ourselves in full sympathy with any of these 
fundamental principles. To Dr. Clements’ conception of “ stimuli ” 
as implied in the first and third of them, we must take serious 
exception. All the factors of the habitat are throughout the book 
spoken of as stimuli, dry air stimulates transpiration, light stimulates 
the plastid in photosynthesis, and light-stimulus causes structural 
changes in the leaf (as in sun and shade leaves), etc. 
All these changes in the plant are of the fundamental nature 
of work done, as evaporation of water or formation of carbohydrates 
or formation of structural parts. If this work is the direct out¬ 
come of energy flowing in from without, then we have a direct 
equation of energy aud work as much as in the expansion of an 
iron bar by heat, and there is no justification for applying the 
special word stimulus to the inflowing energy. Thus it is in trans¬ 
piration and in photosynthesis. When, as in growth-changes, the 
work done is the equivalent of potential energy of the reserves of 
the plant, the inflowing energy only sets the conversion going, and 
the special word stimulus should be reserved for this indirect 
causation. 
Dr. Clements confounds analysis by grouping these two 
categories together as stimuli, and as all producing proportional 
responses. The first produce effects equal and a fortiori pro¬ 
portional to the causes, but are not stimuli ; the second are really 
stimuli and produce effects which are not equivalent to the causes, 
but may happen to be proportional to them if the mechanism 
