104 BIOLOGICAL LECTURES. 



is not an activity differing in kind from the other reactions of 

 these animals. Many sorts of changes in the environment 

 produce a certain characteristic reflex in Paramecia, resulting 

 in their collecting in regions of certain characters and leaving 

 other regions vacant. Among the changes that act thus are 

 chemical changes, and the characteristic groupings of the ani- 

 mals so caused are said to be due to chemotaxis ; they are, 

 however, produced in an essentially similar manner to the 

 groupings produced by other agents. There is a unity under- 

 lying the motor activities of the Paramecia a unity expressed 

 in the fact that the different classes of stimuli produce identi- 

 cally the same reaction. 



To be accurate, however, we must distinguish two less 

 important forms of reaction to stimuli that are not manifested 

 through the characteristic reflex above described. One is 

 thigmotaxis ; this is, however, not a motor reaction, but one 

 characterized chiefly or entirely by a cessation of a part of the 

 usual motion. Again, as previously set forth, it is possible to 

 consider the partially resting condition characteristic of thig- 

 motaxis as the primary condition ; then the ordinary forward 

 motion of the animal will be a motor reaction to a stimulus, 

 since it is induced by a change in the environment. As will 

 be shown, there is sufficient ground in certain other Infusoria to 

 compel us to consider this forward motion as at times a reaction 

 to stimulus ; this, then, is a motor reaction which does not take 

 place through the above-described characteristic reflex. It seems 

 possible that the following represents the true state of the case ; 

 very weak stimuli acting on the resting individual cause the 

 ordinary forward motion ; stronger stimuli produce the above- 

 described motor reflex. 



In view of the means by which chemotaxis is brought about, 

 it becomes more intelligible why the Infusoria may at times 

 collect in regions of injurious substances and avoid at times 

 areas of harmless substances. It is not a matter of attraction or 

 repulsion at all. In the former case the injurious substance 

 merely does not act as a stimulus to cause the motor reflex ; in 

 the second case, the chemical in question, though not injurious, 

 does act as a stimulus. An extended investigation directed 



