156 THEORETICAL BIOLOGY 



mark-organ and action-organ lies the watershed of the whole 

 function-circle. The mark-organ and the action-organ are 

 each of them controlled by a rule ; the one arranges the 

 impressions in the mark-organ, and so creates the indications; 

 the other arranges the effects produced by the action-organ, 

 and so creates the actions. Both rules are focussed accurately 

 on the indication in the external world, the appearance of 

 which is the signal for the indications to arise, and which 

 has then "to be dealt with." The circle forms a unified 

 whole, for, just as in an organism, each part is dependent 

 on the others. The design which connects each part becomes 

 intelligible down to the last detail only when we see the 

 circle as a whole. The receptors are focussed on the typical 

 manifestations of the indicators, whether these be chemical, 

 optical or of some other kind ; and, in virtue of their specific 

 structure, the effectors deal with the indicator in the most 

 effective way. The mark-organs and action-organs are just 

 as nicely focussed on the indicator as are the receptors, and 

 their rules embrace it with scrupulous exactness from the 

 sides both of action and of reaction. 



The diagram given above serves to illustrate the whole 

 of what is done by an animal's nervous system, in so far as 

 this relates to reflexes, plastic actions, or instinctive actions. 

 In the case of reflexes, however, we must assume that the 

 framework of the action-organ is all ready and prepared 

 beforehand ; while with instincts, the rule of the action- 

 organ can still be built up and broken down again. We 

 know a number of cases that are explicable only by super- 

 mechanical regulation, and so prove the intervention of proto- 

 plasm. In contradistinction to these, reflex actions, which 

 are usually of a simple kind, unfold, as it were, automatically. 



But it is characteristic both for reflex and for instinctive 

 actions, that the action-rule reveals itself only in the actions, 

 and in no way enters into the indications and the rules affect- 



