THE HIGHER ORGANISMS 157 



particular series of vibrations, another part to a different series. 

 . . . The sense-organs may be compared with windows each of 

 which opens out into a particular field so as to admit its own series 

 of environmental forces. In each species of animal these windows 

 are arranged in a characteristic way, so as to admit only those forms 

 of energy which are of practical significance to the animal as it lives 

 in its own natural environment." 



These "windows" of Herrick's comprise the organs of 

 special sense touch, taste, scent, sight, hearing, etc., 

 and are recognized as feelers, tongues, noses, eyes, ears, 

 etc. They all admit vibrations to the central nervous 

 system. Without them higher animals would be unable 

 to exist but for a short time, during which they would 

 seem to themselves to float in space in eternal silence and 

 darkness. 



- Nerve-fibre "~ " Nerve - fibre 



Capsule 



- Nerve-fibre 

 _ Nerve-fibre 



Fio. 58. Meisner's corpuscle from man; X 750. (Bohm, Davidoff, and Huber.) 



Touch. The tactile sense can be traced to the irritabil- 

 ity of living substance. It begins without special organs 

 as the phenomenon of thigmotropism. The pseudopods 

 of the rhizopoda are thigmotropic, hence tactile and 

 discriminating. But in composite organization it is not 

 sufficient that the cells shall be equally irritable and 

 similarly impressed by external agents. Division of 



