Literary Notices. yi 



b) After once associating the various parts of a problem, adult 

 rats make only the movements necessary to attain the desired end, 

 while young rats — owing to their superabundant physical activity and 

 lack of muscular control — continue to make useless movements long 

 after adult rats have discarded them entirely. 



c) There is a gradation in the number of useless movements made 

 by rats at different ages. At thirty-five days of age, when physical 

 activity appears to have reached its highest stage, the percentage of 

 useless movements is largest. As the rats grow older this super- 

 abundant activity disappears, and in its place comes direction of ac- 

 tivity. 



Concerning the stages of memory the author writes : 



1. Until the rat has reached the age of twelve days, life to it is 

 simply a matter of pure instinct. Certain movements are made, but 

 these movements are dependent upon the ready-made adjustments of 

 neural and motor elements with which the rat begins life ; intelligence 

 plays little or no part. 



2. At twelve days of age memory is present in a simple form. 



3. From the twelfth to the twenty third day there is a gradual but 

 rapid increase in the complexity of the memory processes until at the 

 latter age psychical maturity is reached. Development after this age 

 is analogous to the development that takes place in a child of ten years 

 as he gradually becomes more and more mature. 



Parts II and III. Having investigated the capacity of rats to 

 learn simple associations, at different stages of development, the author 

 proceeded to make a careful histological study of the changes which 

 occur in the nervous system from birth to maturity in order that he 

 might be able to correlate the psychical and neural conditions and 

 definitely determine whether associations are dependent upon the me- 

 dullation of nerve fibers. As a result of this work Dr. Watson con- 

 cludes : (i) that the "meduUated fibers in the cortex of the rat are not 

 a conditio sine qua ?ion of the rat's forming and retaining definite associ- 

 ations, and (2) that the complexity of the psychical life increases much 

 more rapidly than does the medullation process in the cortex, psychical 

 maturity being reached when approximately only one-fifth of the total 

 number of fibers in the cortex are medullated." 



Instead of speculating about the general significance of medulla- 

 tion the author very wisely confines himself to the discussion of his 

 own particular facts. The experimental work is clear cut and decisive, 

 and if one sometimes feels that fewer words might have sufficed and 

 and space been saved by the condensation of results into tables, the 

 excellent summaries more than compensate for the lengthiness of the 

 descriptions. Dr. Watson has done a valuable piece of work in a 

 field which has been open thus far for the theorizing of neurologists 

 and psychologists. robert yerkes. 



