3o8 THEORETICAL BIOLOGY 



during the breeding-season there comes in a changing of the 

 steering-gear which is of far-reaching importance for the 

 selection of indications. Animals endowed with plastic 

 power frequently suppress an indication, though it be oft 

 repeated, by raising their threshold-level against it ; and this 

 is especially obvious in the case of actions based on 

 experience. 



In many animals an inner rhythm, consisting of waking 

 and sleeping, strives to faU in with the outer rhythm of the 

 life-path that expresses itself in the alternation of day and 

 night. By raising the threshold during sleep, all indications 

 can periodically be suppressed. In the spotted dogfish, 

 which rest aU day and seek their prey by night. Beer found 

 a periodic opening and closing of the pupil, which could be 

 demonstrated in animals kept continually in the dark. The 

 rhythm of ebb and flow is responded to in littoral sea-anemones 

 by a periodic change in the reflex activity. In these animals 

 also Bohn was able to demonstrate that there was an inner 

 rhythm, which persisted for days in anemones kept in an 

 aquarium. 



The rhythmical change of tone in an apparatus, compelling 

 it to change of work, is an arrangement unknown in machines, 

 and can be achieved only by means of a further mechanical 

 device. The framework of animals, characterised by the 

 presence of a protoplasmic matrix, makes the rhythmic 

 change seem less mysterious. Actually, those who have con- 

 cerned themselves more closely with the question, see in the 

 protoplasm the cause of this phenomenon. 



We might speak of a chemical tide-change in the proto- 

 plasm, excited more or less by the external rhythm. These 

 matters still lie so far beyond the possibility of actual inves- 

 tigation that we are reduced to mere conjecture. All that is 

 certain is that there must be a rhythmically active impulse- 

 system at the back of this phenomenon also, furnishing the 



