INQUIRY INTO CAUSES OF AUTONOMOUS MOVEMENTS 3 II 



retardation of conductivity and excitability, which was, how- 

 ever, gradually recovered, and there were produced a second 

 and then successive periodic responsive movements. 



In the hydraulic model, if the capacity of the reservoir be 

 small, the cessation of the water-supply will cause an immediate 

 cessation of the rhythmic movement of the indicating lever. 

 In BiopJiytum, similarly, we found that the periodic response 

 continued as long as energy was supplied, and that the 

 movement soon stopped on the stoppage of the supply of 

 external energy. But if the capacity of the reservoir be great, 

 the accumulation may be sufficient to maintain the oscillation 

 for a considerable length of time, even after the main supply 

 is cut off. And we see that in Desnwdium the responsive 

 movements continue in a persistent manner, though the 

 immediate source of stimulation be interrupted. A similar 

 difference in the persistence of after-effects, depending on 

 relative capacities for storage of energy, is seen in the two 

 classes of inorganic substances, which are distinguished as 

 fluorescent and phosphorescent. In the first, the responsive 

 emission of light caused by a preceding excitation is 

 extremely short-lived ; but in the latter it continues long 

 after the light stimulus has ceased to act. Thus Desmodium 

 may continue to exhibit rhythmic movements, though not at 

 the moment exposed to the marked action of any special 

 source of stimulus. But for the display of long-continued 

 rhythmic movements it should previously have absorbed a 

 considerable amount of energy from an external source — in 

 other words, it must have been exposed to those circum- 

 stances which produce a favourable tonic condition. 



We have thus obtained some insight into that very ob- 

 scure phenomenon which is known as the after-effect. By 

 the inertia of the organism there is a certain loss of time 

 before response begins to take place, and this determines the 

 latent period. But when the stimulus has already initiated 

 movement, the responding organ will, through the same inertia, 

 continue to show this movement even when the stimulus has 

 ceased to act. There is another factor, however, which 



