GEOTAXIS 29 
Crustaceans are able to detect food at some distance and 
direct themselves toward it, but one may question if such 
phenomena should be classed under the head of true chemo- 
taxis. Similarly with the olfactory reactions of ants. 
These insects follow a scent track with considerable accuracy. 
In this way they may be guided to food discovered by other 
ants and find their way back to the nest. The tendency to 
follow these tracks is doubtless instinctive, and so also is the 
action of a dog in following the trail of a rabbit. The be- 
havior of the dog is on a much higher level than a mere 
tropism, and it is probable that the behavior of the ant is 
also, but to a less degree. But where to draw the line 
between such actions and chemotaxis proper is perhaps 
capable of only an arbitrary decision. 
GEOTAXIS 
Gravity exercises a directive effect upon the position and 
movements of many animals while in other forms it has little 
orienting power. The protozoans Euglena and Chlamy- 
domonas commonly swim upward in the dark as well as in 
the light, but this reaction is checked at a low temperature 
of 5° or 6° C. Massart studied the geotactic movements of a 
number of unicellular plants and animals by placing them in 
a capillary tube open at either end so as to render the supply 
of oxygen the same above and below. He found that the 
sense of the geotactic response varies in allied species of 
the same genus, as for instance, Spirillum some of which 
are positive and some negative under the same conditions. 
The sense of the response could be changed in some cases by 
temperature. Thus Chromulina, which is negative at 15° 
to 20° C., becomes positive at a temperature of 5° to 7° C. 
The geotaxis of Paramcecium is quite variable and often 
quite feeble. Generally there is a tendency to swim up- 
