312 THEORETICAL BIOLOGY 



The rate at which the extension and contraction in the 

 muscles of the limbs alternate, depends in considerable degree 

 on the obstacles on the path at the time, obstacles which 

 the muscles must overcome by their contraction. If an 

 internal beat were trying here to regulate the movements, 

 this would only add to the difficulties. 



We find an interiorly determined beat only in those effector 

 organs that have always the same obstacle to overcome, 

 such as the hearts of vertebrates, the margin of the umbrella 

 of jelly-fishes and the wings of insects. The hearts of inverte- 

 brates move according to the general law of tension ; their 

 beat, therefore, is not in constant dependence on the amount 

 of blood they contain. The stroke of the wing in a bird is 

 regulated all the time by the receptors, while in insects it is 

 only the beginning and the end of the rhythmical wing-beat 

 that depend on these ; the rhythm itself is quite automatically 

 exerted from the wing-muscle centres. 



The effector organs which preserve their own beat-rhythm 

 have for this an arrangement of their own, which is expressed 

 in the so-called " refractory period." The refractory period 

 rhythmically lowers to zero the threshold-value for the regular 

 waves of excitation flowing to the points where the nerves 

 enter the muscles. In what the arrangement consists has 

 not yet been ascertained. 



The beat-rhythm in the receptor portions of the steering- 

 mechanism is of quite another kind. It serves to transform 

 the external rhythms in the surrounding-world into an indica- 

 tion, and in this way to fit these into the world-as-sensed. 

 Let us imagine that a chemical tide-change of unequal rate 

 continually controls the mark-organ of an animal ; then we 

 should see in this the means by which the animal would be 

 able to refer external rhythms to its own beat, and thereby 

 connect them into an indication. 



As we know, the power to form indications is associated 



