VARIATIONS IN REACTION TO LIGHT. q 
I consider that the most probable explanation of this is the constant 
association, in the natural habitat of the organisms (the sea), of swimming 
upwards towards the hight when positively phototactic, and downwards 
towards the darker regions of water when negatively phototactic. 
In addition to the interest of this association on its own account, it 
seems to me to be valuable as a sign that the light not only affects the 
sensitive area on which it acts, but also indirectly affects the whole 
organism, the chemical changes set up at the sensitive area communicating 
changes to the whole organism, which stimulate it and cause it to rise or 
sink in the medium. 
Loeb, working with Gammarus, found that traces of acid made the 
organisms more strongly positive, and traces of alkali tended to produce a 
negative heliotropic effect. I have not been able to obtain similar results 
with hydrochloric acid, or caustic soda, in Nauplius, although both 
reagents were pushed to the limits compatible with life, viz., 45 normal. 
The organisms in the dishes to which either acid or alkali was added 
seemed to behave exactly like the untreated control. I do not, however, 
consider this any contradiction of Loeb’s results, since the organisms used 
were different. Moreover, Loeb’s results are such as would be expected 
from the knowledge that alkalies, within the compatible range, excite the 
activity of living matter, while acids depress it. For, if we regard the 
hght effect as producing an increased chemical activity, then the optimum 
value of reaction, at which the change would occur from positive to 
negative as the intensity of the illumination was increased, might be 
expected to be reached sooner in the case of an organism already made 
hyperactive by alkali than in the case of an organism where the activity 
was depressed by added acid. 
Throughout the whole series of my experiments I have consistently 
found that the phototaxis is positive with very feeble illumination, and 
becomes negative as the strength of the light is increased. Further, 
continued illumination, either by diffuse daylight or by a very bright 
artificial illumination, causes an increasing number of organisms to 
1, The limit of acidity or alkalinity compatible with life seems to have nearly the above 
value for all unprotected minute organisms of either vegetable or animal origin. 
