134 THEORETICAL BIOLOGY 



the physiologist follows, on its path through space, the effect 

 proceeding from the object. In studying an animal, he will 

 trace the air-waves to the ear, and there observe their trans- 

 formation into nervous excitation ; in like manner, he will 

 follow a light-ray as an etheric wave to the eye, investigate 

 its refraction by the optical media, note the production of 

 images on the retina, and discover the chemical transformation 

 into nervous excitation that takes place in the rods and 

 cones. In the same way, he will follow the chemically active 

 particles in the media of air and water as far as the smell- 

 receptors, and their effect on the nerve-endings of the olfac- 

 tory nerves. And so he will deal with the other sense 

 impressions. 



In the course of this study it becomes evident that, out- 

 side the animal under observation, numerous influences pro- 

 ceed from the object that are not taken up, because a certain 

 intensity of the external influence is required if a nerve is to 

 be excited. Physical and chemical processes in the outer 

 world must exceed a certain threshold if they are to act as 

 stimuli affecting the nervous system of the animal. This 

 threshold can be raised or lowered by special means in the 

 animal's nervous system. Moreover, by their anatomical 

 structure, the receptors are obliged to admit only those 

 stimuli that are suitable for them. 



By the structure of its receptors, every animal is cut off 

 from a great number of physical and chemical influences 

 coming from the outer world, and it is only through stimuli 

 that the outer world gets in touch with the nervous system. 



If we take the anatomical structure of the receptors as 

 given, this whole process can be investigated by purely 

 physical and chemical methods, and that is just what physi- 

 ology has to do. 



When an animal meets with different scimuli, the physi- 

 ologist will be inclined to assume that these make different 



