10 ROBERT M. YERKES 



letns, however, are completely soluble by an organism of excellent 

 ideational ability. For the human subject, the solution of the 

 easiest problem of all requires almost no effort, whereas even 

 moderately difficult problems may require many repetitions 

 of effort and hours or days of application to the task. In each 

 case, the solution of the problem depends upon the percept: on 

 of a certain constant relation among a series of objects to which 

 the subject is required to attend and respond. Such relations 

 are, for example, secondness from one end of the group, middle- 

 ness, simple alternation of ends, or progressive movement by 

 constant steps from one end of a group to the other. 



It is possible to present such relational problems by means 

 of relatively simple reaction-mechanisms. In their essential 

 features, all of the several types of multiple-choice apparatus 

 designed by the writer and used either by him or by his students 

 and assistants are the same. They consist of a series of pre- 

 cisely similar reaction-devices, any one or all of which may be 

 used in connection with a. given observation. These reaction- 

 mechanisms are so chosen as to be suited to the structure and 

 action-system of the animal to be studied. For the human 

 being the mechanism consists of a simple key and the total 

 apparatus is a bank of keys, with such electrical connections 

 as are necessary to enable the observer to obtain satisfactory 

 records of the subject's behavior. Let us suppose the bank of 

 keys, as was actually the case in my first form of apparatus, 

 to consist of twelve separate reaction-mechanisms; and let us 

 suppose, further, the constant relation (problem) on the basis 

 of which the subject is required to react to be that of middle- 

 ness. It is evident that in successive trials or experiments the 

 keys must be presented to the subject in odd groups, the possi- 

 bilities being groups of 3, 5, 7, 9, or 11. If for a particular ob- 

 servation the experimenter wishes to present the first three 

 keys at the left end of the keyboard, he pushes back the remain- 

 ing nine keys so that they cannot be operated and requires the 

 subject to select from the group of three keys the one which on 

 being pressed causes a signal to appear. It is of course the 

 clearly understood task of the subject to learn to select the cor- 

 rect key in the group on first trial. This becomes possible only 

 as the subject observes the relation of the key which produces 

 the desired effect to the other keys in the group. On the com- 



