I40 THEORETICAL BIOLOGY 



requires considerable skill in observation and experiment ; 

 but the most important thing is to formulate the question 

 rightly, and this can come only from a sufficient knowledge of 

 the basic problems. 



THE HIGHER GRADES OF THE WORLDS-AS-SENSED 



As soon as outlines of bodies appear as indications, the 

 picture presented by the world-as-sensed alters fundamentally, 

 for now juxtaposition in space begins to become more and 

 more important. It is true that in the lower animals, such as 

 sea-urchins and molluscs, a stimulus coming from the left is 

 responded to otherwise than one coming from the right, for, 

 in the one instance, the effector organs of the right side of the 

 body respond, and, in the other, those of the left side. 



But the indication itself remains the same, and in its 

 composition shows no sort of spatial differentiation. Only 

 when spatial distinctions appear in the indication itself, can 

 we speak of a higher grade of sensed-world. The eyes of 

 insects have the power of transferring to the central nervous 

 system in a schematic form definite spatial arrangements of 

 the pictures appearing on their retina, to which we shall 

 refer when dealing with the inner world of animals. For the 

 study of the world-as-sensed it is sufficient to determine what 

 sort of outlines, and in what degree of exactness, are employed 

 as indications. 



It is worth noting that, in an insect, outlines are not 

 swept by glancing movements of the eye, but by a stationary 

 retina. It is very difficult for us to determine in how far our 

 eye, when stationary, estimates outlines. In any case, when 

 the retina is not moving, it deals only with surfaces, and not 

 with solid bodies. 



It is very important, but very difficult, to set about these 

 experiments with animals in a really critical spirit. We are 



