142 THEORETICAL BIOLOGY 



only in such cases do we get an unequivocal response through 

 the special reactions of the animal. 



As has already been pointed out, it is not at all necessary 

 that the indications of an animal should reach the same . 

 height in each of its function-circles. 



As a rule, in the enemy-circle a mere movement will serve, 

 whereas in the prey-circle even the outlines may have this 

 value. The disputes between investigators hitherto depend 

 on this difference, some maintaining that colours exist in the 

 sensed-world of the lower animals, and others saying that they 

 do not. 



Among the Crustacea, the indications of the prey-circle 

 seem to be of a purely chemical nature, while those of the 

 enemy-circle are optical. 



The world-as-sensed undergoes an important enlargement 

 when indications appear for the movements of the animal's 

 own limbs. It is only among vertebrates that sensory nerves 

 have been demonstrated with certainty in the muscles. And 

 it is only when nerves of this kind appear that we can speak 

 of a new function-circle, passing through the animal's own 

 body. 



From experiments on vertebrates in which the sensory 

 roots of the spinal cord have been severed, we know some- 

 thing about this function-circle. There can be no doubt 

 that it is only when the animal's own body in movement 

 becomes an indication that a sharp line can be drawn in the 

 world-as-sensed between the subject and the outside world. 

 This separation of the two is quite lacking in the lower 

 animals ; their own subject has no indications, for, as we 

 shall see later on, in dealing with the lower animals even 

 pain must be ruled out. 



The highest grade of world-as-sensed is reached when 

 implements themselves become indications. Unfortunately, 

 the American workers who have taken up this question have 



