596 THE POPULAR SCIENCE MONTHLY. 



gins its work that is, there is an adjustment between the internal 

 relation of a certain sensation to the digestive process and the exter- 

 nal relation between the presence of foreign matter in the stomach 

 and food. 



There is a relation between the sudden approach of a body toward 

 the eye and danger to that organ, and to this relation the nerves and 

 muscles are adjusted. There is a relation between a peculiar "cluck" 

 and the presence of the mother-hen, and to this I'elation the cliick 

 responds. There grows up in the mind of the soldier a connection 

 between the word "attention" and a particular attitude, and when he 

 feels the sensation caused by the command, he at once, and uncon- 

 sciously, performs the muscular movements necessary to assume that 

 position; that is, there is an adjustment between the internal relation 

 of a certain sensation to certain muscular movements and the exter- 

 nal relation of a certain command to the necessity for assuming a 

 given position. The sailor learns that there is a relation between cer- 

 tain signals and stormy weather, and to this relation his actions are 

 adjusted. Finally, General Meyer finds that there is a relation between 

 low barometric pressure in certain parts of the country and a liability 

 to storms in other regions, and to this external relation he adjusts his 

 actions. 



The kind of external change to which an organism may respond 

 of course varies greatly in diiferent cases, both in constancy and com- 

 plexity. The Venus's-flytrap is adjusted only to relations between 

 objects on the surface of the leaf and in contact with it, but an animal 

 with even rudimentary organs of sense may respond to changes which 

 occur at a distance. Even the simple capacity to distinguish light 

 from darkness is enough to enable an animal to perceive a distant 

 body between it and the sun, and to adjust its actions accordingly. 



As we ascend to creatures having more developed eyes, we find an 

 increase in tlie sphere of surrounding space throughout which external 

 relations can establish corresponding internal relations. A slight con- 

 vexity of the epidermic layer lying over the sensitive tract first serves, 

 by concentrating the rays, to render appreciable less marked varia- 

 tions in the quantity of light, and thus brings into view the same 

 bodies at a greater distance, and smaller or less opaque bodies at the 

 same distance. From this point upward, through the various types 

 of aquatic creatures to the higher air-breathing creatures, we trace, 

 under various forms and modifications, a complicated visual apparatus 

 and a widening space through which the correspondence extends. It 

 is needless to go into details. Hypotheses and illustrations aside, it 

 is obvious that from the polyp, which does not stir till touched, up 

 to the telescopic-eyed vulture or the far-sighted Bushman, one aspect 

 of progressing life is the greater and greater remoteness at which visi- 

 ble relations in the environment produce adapted relations in the 

 organism. The extension of the correspondence in space does not end 



