Page:Popular Science Monthly Volume 85.djvu/34

30 of heat and cold and if one end of a dish containing paramecia is heated and the other end is cooled by ice, the paramecia collect in the region somewhere between these two extremes (Fig. 19). Jennings, by studying carefully the behavior of single individuals, established the fact that this apparently intelligent action is due to differential sensitivity and to the single motor reaction of the animal. If in the course of its swimming a Paramecium comes into contact with an irritating substance or condition, it backs a short distance, swerves toward its aboral side, and goes ahead in a new path; if it again comes in contact with the irritating

 ''Paramecium. A'' is a solid object or other source of stimulation. 1-6, successive positions occupied by the animal. The rotation on the long axis is not shown. (After Jennings.)

conditions this reaction is repeated, and so on indefinitely until finally a path is found in which the source of irritation is avoided altogether. In short, Paramecium continually tries its environment, and backs away from irritating substances or conditions. Its apparently intelligent reactions are thus explained as due to a process of "trial and error."

The behavior of worms, star-fishes, crustaceans, mollusks, as well as of fishes, frogs, reptiles, birds and mammals, have been studied and in all cases it is found that their method of responding to stimuli is not at first really purposive and intelligent but by the gradual elimination of useless responses and the preservation (or remembering) of useful ones the behavior may come to be purposive and intelligent.

Thorndike found that when dogs, cats and monkeys were confined