Wager: Movements of Aquatic Micro-Organisms. 177 
other organisms are repelled by the light. This is said to be 
due to a too great intensity of the light. But this is obviously 
not the only factor in the response, for light of the same in- 
tensity may at one time cause repulsion, at another time 
attraction, in the same individuals. One of the determining 
factors is temperature. If the intensity of the light remains 
constant we find that in bright sunlight at a low temperature 
Euglense are negatively heliotactic ; if the temperature is 
increased some of them become positively heliotactic, and 
if the temperature is still further increased more and moie of 
them become positive, until finally at a much higher tempera- 
ture they all become positively heliotactic. If we now 
gradually lower the temperature, the intensity of the light 
remaining the same, the Englense pass through these varying 
stages of positive and negative responses in the reverse order, 
until at a low temperature they again all become negatively 
heliotactic. The experiment may be repeated many times in 
succession with the same culture of Euglena. 
When the temperature remains constant, we find that at a 
low temperature and low intensity of light Euglenae are positive, 
but if the temperature remains the same, as the intensity of 
the light is increased they gradually become more and more 
negative, until finally they are all negatively heliotactic. 
The general conclusion at which we arrive from these 
experiments is that when the intensity and temperature are 
very disproportionate we get negative heliotaxis, but that 
when the two are more nearly proportional we get positive 
heliotaxis. Thus at a low temperature and high intensity of 
light, or at a high temperature and low intensity of light the 
Euglenae are negative ; at a high temperature and high in- 
tensity, or at a low temperature and low intensity of light they 
are positive. 
In support of his view that the movements of organisms 
towards the source of light are purely mechanical or auto- 
matic, Loeb lays a good deal of stress upon the fact that they 
continue to move towards the source of light ‘ even if in so 
doing they must pass from the light into the shade.’ The 
evidence adduced in support of this statement is, however, 
not conclusive. It is quite true that organisms may pass from 
a stronger into a weaker light, but it has not been clearly 
shown that this may not be due to a preference for the weaker 
illumination. It can for example be easily demonstrated by 
experiments with Euglena, Chlamydomonas and barnacle 
larvae, that they do not always pass into the shade, but that 
whilst a number of them from the same culture may do so, a 
large proportion of them may remain in the stronger light, 
notwithstanding the fact that the direction of the light rays 
remains the same in both. Thus we expose a shallow vessel 
1914 June 1. 
