Page:Popular Science Monthly Volume 85.djvu/36

32 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 stimuli, external or internal in origin. These stimuli are in the main, if not entirely, energy changes outside or inside the organism. In lower organisms as well as in the germ cells and embryos of higher animals the possible number of responses are few and prescribed owing to their relative simplicity, and the response follows the stimulus directly. In more complex 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 internal 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 concealed 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 treatment 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.

(b) Responses may also be modified through compulsory limitation of many possible responses to a particular one, and the consequent formation 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