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. 



Paramcecia, 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 paramcecia collect around it in crowds, 

 and eat the bacteria ; but they will collect in the same wav 

 around almost any solid. On coming in contact with bodies 

 having a certain physical texture, the cilia covering the para- 

 mcecium 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 paramcecia 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- 



