TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
Thus, we shall see that with the same condition and previous treatment 
of the organism, the reaction varies and becomes positive or negative 
with varying intensities of the lhght-stimulus, and, secondly, with the 
same constant intensity of light-stimulus, the reaction varies when the 
previous history and condition of the reacting organism have been 
artificially varied. That is to say, the light induces chemical alterations 
in the cell, and the nature and amount of the chemical changes vary with 
the two factors, the condition of the cell at the time, and the intensity of 
the incident hght. 
It has been clearly pointed out by Loeb that the orientations or 
tropisms of sessile organisms, and the movements of free organisms 
towards or away from light, are essentially the same in character, the free 
organism being first orientated and then, by the action of its locomotor 
organs, carried in either direction according to the sense of the previous 
orientation. | 
This is a fundamental observation which to a certain extent unifies © 
the problem, but there still remain the questions of why the hight induces 
orientation, the conditions under which orientation varies with the 
condition of the organism and the strength of stimulation, and also the 
remarkable fact that in higher organisms at any rate there is developed 
what might be described as resistance to orientation, so that the organisms 
accumulate either at the proximal or distal point to the ight and yet lie 
in all possible planes of orientation, and, further, that they move about 
within a certain zone in all possible directions. 
It is in fact self-evident, and may be taken as axiomatic, that there 
must have been a certain degree of orientation, or steering, or the 
organisms would never have been able to move either to or from the light. 
But this, it is to be observed, is quite different from the organism being 
turned round when the movement first begins, being definitely held there 
by the influence of the light in a fixed plane, and then as a result moving 
towards or away from the light. 
The experiments to be recorded later show clearly that there is no 
such fixed or rigid orientation keeping the organisms in a constant plane, 
but rather a continually directed control bringing the organism back 
