102 BIOLOGICAL LECTURES. 



that swimming backward does, as a matter of fact, usually carry 

 the animal away from the source of diffusion of the chemical, and 

 turning toward the aboral side before swimming forward again 

 will, as a rule, if repeated, finally carry the animal in such a direc- 

 tion that it does not again come against the source of stimulus. 

 But these are, from the physiological standpoint, matters of 

 accident ; the animal conducts itself in the same way whether 

 the source of stimulus has this usual position at the anterior 

 end of the animal or not. The direction of motion after a 

 chemical stimulus, then, has no relation to the position of the 

 chemical substance. We cannot say, therefore, that the Para- 

 mecia are repelled by any chemical substance just as we 

 were compelled to conclude that they are not directly attracted 

 by any chemical substance. 



We find, then, that the effect of chemicals on Paramecia is 

 not to attract or repel them, but simply to cause a certain set 

 formula of movements. Such a set formula of movements, 

 "touched off," as it were, by stimuli of various sorts, may be 

 called a reflex. In returning now to the question of how the 

 apparent attraction of the Paramecia toward certain substances 

 that is, the fact that they collect in drops of certain sub- 

 stances can be brought about through such a reflex, it is neces- 

 sary to recall certain general facts in regard to the nature of 

 reflexes. First, any change in the environment that can be 

 perceived by the organism may "touch off" such a reflex. 

 Second, the character of the reflex has no necessary relation to 

 the nature of this external change, so that of a given kind of 

 change it cannot be predicted beforehand whether it will cause 

 the reflex or not, and changes of opposite character may pro- 

 duce the same reflex. 



The mechanism of the gathering together of the Paramecia 

 into a drop of some weak acid is then as follows : When the 

 Paramecium passes from the surrounding fluid into the acid 

 solution there is, of course, at the moment of crossing the 

 boundary of the drop a change in its environment. Whether 

 this change will cause the characteristic reflex or not is impos- 

 sible to predict, that depending upon the internal mechanism 

 of the organism ; as a matter of fact we find that it does not 



