138 THEORETICAL BIOLOGY 



organisation as a whole that it consists of all the functional 

 units which we call organs. These are divided up into sub- 

 sidiary organs, on which devolve the exercise of part-functions, 

 while collectively they carry out the function of the whole. 

 Thus the receptor nerves (those which receive stimuli and 

 conduct excitation) all form separate part-organs, which 

 open into another organ, the nervous network for collecting 

 stimulation. This network, on its part, sends out intra- 

 central fibres to the motor organs of the nervous system. 



The part-organs collectively form a closed organ, which, 

 in virtue of its function, we may call a mark-organ, since its 

 duty is to create the indications that are of importance to 

 the animal. 



If the mark-organ embraces the entire central network of 

 all the receptor nerves, the animal has only one single indica- 

 tion : this is the case with the lower animals. 



In higher animals, where the mark-organ includes the 

 receptor nerves in the most various combinations, the same 

 receptor nerves can enter into connection with different nerve- 

 networks. Then different indications in the outer world 

 exhibit certain similar properties. 



Finally the mark-organs may serve to unite into one 

 indication, not merely simultaneous impressions, but impres- 

 sions received at different times. And this is a super- 

 mechanical faculty. 

 ^ In animals that are little centralised, such as the sea- 

 urchin, the mark-organs embrace without distinction the 

 localised nerves lying alongside one another ; in such a case, 

 the indications are separated from one another only by space. 

 In the higher animals, with highly developed sense-organs, 

 the organs for the indications receive nerves, the local grouping 

 of which retains its specific irritability. In such cases, the 

 indications differ from one another as regards content. 



Even if we had very exact knowledge concerning the 



