SENSATION 



(more recently taxis, of. chapter xii. of the Riddle). We 

 may illustrate it from the simplest case of chemical com- 

 bination. When we rub together sulphur and mer- 

 cury, two totally different elements, the atoms of the 

 finely divided matter combine and form a third and 

 different chemical body, cinnabar. How would this 

 simple synthesis be possible unless the two elements feel 

 each other, move towards each other, and then unite? 

 We find universally distributed in nature the sensation 

 of the mechanical stimulus of gravitation, the most 

 comprehensive statement of which is given in Newton's 

 law of gravity. According to this fundamental and all- 

 ruling law, any two particles of matter are attracted in 

 direct proportion to their mass and inverse proportion 

 to the square of their distance. This form of attraction, 

 also, can be traced to a "sensation of matter" in the 

 mutually attracting atoms. The local sensation that 

 any body provokes by contact with the surface of an 

 organism is felt as pressure (baros). A stimulus that 

 causes this pressure alone brings about a counter-press- 

 ure as a reaction, and an effort to neutralize it, the 

 pressure-movement (barotaxis or barotropism). Sensi- 

 tiveness to pressure or the contact of solid bodies is 

 found throughout the organic world ; it can be proved . 

 experimentally among the protists as well as the histona. 

 Special sense-organs have been developed in the skin of 

 the higher animals as the instruments of this pressure- 

 sense (barsesthesis) in the form of tactile corpuscles; 

 they are most numerous at the finger-tips and other 

 particularly sensitive parts. In many of the higher 

 animals there is a fine sense of touch in the feelers or 

 tentacles, or (in the higher articulates) in the horns or 

 antennae. Moreover, these tactile and prehensile organs 

 are also very widely found among the higher plants, 

 especially the climbing plants (vines, bryony, etc.). 

 Their slender creepers, which roll out spirally, have a 



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