46 JOSEPH U. YARB ROUGH 



themselves mediate the association. However, in our experi- 

 ments with animals these objections are not valid, for few be- 

 lieve that white rats master problems by the use of memory 

 images and concepts, c) A third and perhaps a more vital ob- 

 jection to this theory has been made by Carr; namely, that while 

 one can easily conceive of the persistence of a neural activity 

 for a short interval of time on the principle of inertia, yet the 

 assumption of an indefinite continuance rather taxes one's cre- 

 dulity. For this reason a fourth type of explanation is per- 

 haps the most acceptable. 4) This, the last theory advanced, 

 explains the connection formed between the two stimuli in terms 

 of the decreased resistance of the motor center. The connection 

 is mediated by so decreasing the resistance of the motor center 

 that the one term will become the adequate motor outlet for the 

 nervous impulse aroused by the other. Such a conception is 

 based upon the well known and generally accepted theory of 

 synaptic resistance. The two theories have in common two 

 assumptions: i) Each sensory impulse tends to follow that 

 motor path which for the time offers the minimum of resistance. 

 2) The resistance of any center is decreased by an increase in 

 metabolism in that center. If the resistance of one motor center 

 has by continued practice been so reduced that its readiness for 

 response is much greater than that of any other motor outlet, 

 there are no reasons why an effective association cannot be 

 formed over a considerable interval of time. 



Since there was no observable third factor mediating the asso- 

 ciation in our work and since survival of the memory image 

 most likely is not a factor in the behavior of the white rat, there 

 remain but two possible explanations for our results: namely, 

 either the theory of the "akoluthic" phase or the reduced resist- 

 ance of the motor center. 



2. Our discussion has thus far been concerned with the 

 phenomenon of successive association when the two members to 

 be associated were presented in a certain definite temporal rela- 

 tion. We shall now examine successive association when formed 

 under the same conditions except that the order of the presenta- 



