Tropisms and Instincts as Adaptations 397 
on its way. If an individual were put into an alkaline drop, 
it would leave it, because it would not react when it passed 
from inside the drop into the surrounding water. 
Unicellular animals react to other things besides differences 
in the chemical composition of different parts of a solution. 
In many cases they react to light, swimming toward or away 
from it according to whether they are positively or negatively 
heliotropic. If they are positively heliotropic, and while 
swimming run into a shadow, they react as they would on 
coming into contact with a drop of acid. Since they rotate as 
they swim forward, we cannot explain their orientation as in 
the case of other animals that hold a fixed vertical position. 
If we assume that the two ends of the body are differently 
affected by the light, for which there is some evidence, we 
can perhaps in this way account for their turning toward, or 
away from, the source of light. 
Changes in the osmotic pressure of the different parts of 
the fluid, mechanical stimulation produced by jarring, ex- 
tremes of heat and of cold, all cause this same characteristic 
reaction in Paramcecium; and this accounts for their be- 
havior toward these agents that are so different in other 
respects. 
Paramoecia, as well as other protozoans, show a contact 
response. They fix themselves to certain kinds of solid 
bodies. If, for example, a small bit of bacterial slime is put 
into the water, the parameecia collect around it in crowds, 
and eat the bacteria; but they will collect in the same way 
around almost any solid. On coming in contact with bodies 
having a certain physical texture, the cilia covering the para- 
moecium stop moving, only those in the oral groove continu- 
ing to strike backward. The animal comes to rest, pressed 
against the solid body. If one or more parameecia remain in 
the same place, they set free carbon dioxide, as a result of 
their respiratory processes. There is formed around them a 
region containing more of this acid than does the surround- 
