32 TEE POPULAR SCIENCE MONTHLY 



velopment and from relatively simple beginnings. The will of the child 

 has developed out of something which is far less perfect in the infant 

 and embryo than in the child. Observations and experiments on lower 

 animals and on human beings, as well as introspective study of our own 

 activities, appear to justify the following conclusions : 



(1.) Every activity of an organism is a response to one or more stim- 

 uli, external or internal in origin. These stimuli are in the main, if not 

 entirely, energy changes outside or inside the organism. In lower organ- 

 isms as well as in the germ cells and embryos of higher animals the pos- 

 sible number of responses are few and prescribed owing to their relative 

 simplicity, and the response follows the stimulus directly. In more com- 

 plex organisms the number of possible responses to a stimulus is greatly 

 increased, and the visible response may be the end of a long series of in- 

 ternal changes which are started by the original stimulus. 



(2.) The response to a stimulus may be modified or inhibited in the 

 following ways : 



(a) Through conflicting stimuli and changed physiological states 

 (due to fatigue, hunger, etc.). Many stimuli may reach the organism 

 at the same time and if they conflict they may nullify one another or the 

 organism may respond to the strongest stimulus and disregard the 

 weaker ones. When an organism has begun to respond to one stimulus 

 it is not easily diverted to another. Jennings found that the attached 

 infusorian, Stentor, which usually responds to strong stimuli by closing 

 up, may, when repeatedly stimulated, loosen its attachment and swim 

 away, thus responding in a wholly new manner when its physiological 

 state has been changed by repeated stimuli and responses. Whitman 

 found that leeches of the genus Clepsine prefer shade to bright light, 

 and other things being equal they always seek the under sides of stones 

 and shaded places ; but if a turtle from which they normally suck blood 

 is put into an aquarium with the leeches, they at once leave the shade 

 and attach themselves to the turtle. They prefer shade to bright light 

 but they prefer their food to the shade. The tendency to remain con- 

 cealed is inhibited by the stronger stimulus of hunger. On the other 

 hand he found that the salamander, Necturus, is so timid that it will not 

 take food, even though starving, until by gradual stages and gentle treat- 

 ment its timidity can be overcome to a certain extent. Here fear is at 

 first a stronger stimulus than hunger and unless the stimulus of fear can 

 be reduced the animal will starve to death in the presence of the most 

 tempting food. 



(&) Responses may also be modified through compulsory limitation 

 of many possible responses to a particular one, and the consequent forma- 

 tion of a habit. This is the method of education employed in training 

 all sorts of animals. Thus Jennings found that a starfish could be 

 trained to turn itself over, when placed on its back, by means of one 

 particular arm simply by persistently preventing the use of the other 



