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THE AMERICAN NATURALIST [Vol. XLII 



the reader would appreciate better their argument as to the ex- 

 clusive role of the inner senses. One is slightly inclined to feel 

 that the authors are making the common mistake of weakening 

 the effect of a demonstration of the unsuspected great importance 

 of a certain factor, by endeavoring to maintain that it is the only 



Another factor in rat behavior is dealt with in the paper of 

 Slonaker. 3 Watson in his earlier work had found that young 

 rats (25 to 30 days old) learn certain operations more quickly 

 than do the adults, and this appeared to be due to the fact that 

 the young were more active. They tried, in a short time, all sorts 

 of movements, and were therefore likely to hit quickly upon the 

 "right" ones. Slonaker makes a careful study of the compara- 

 tive amounts of activity in rats of different ages, in order to see 

 how far this is correlated with the quickness of learning. A 

 revolving cage was used, of such a character that the activity of 

 the rats was automatically recorded. One such experiment, with 

 several rats, lasted 25 days ; another 57 ; another 60. It was 

 found that, while there are great differences in individuals, both 

 the very young rats and the old ones (age about a year) were 

 comparatively inactive; the period of greatest activity falls in 

 the age between 87 and 120 days. No close relation was evident 

 between these results and those of Watson on quickness in 

 learning. 



All together, the work of the Chicago laboratory has of late 

 been directed with unusual precision on a well-defined single line 

 of research. And hand in hand with this work on behavior have 

 gone studies on the nervous system, growth and life history of the 

 rat, many results of which have been published from the neuro- 

 logical laboratory. Such a unified and intensive series of in- 

 vestigations may well serve as a model of the best method for 

 making solid and permanent advance in comparative psychology. 



H. S. Jennings. 



•Slonaker, J. K. The Normal Activity of the White Eat at Different 

 Ages. Journ. Comp. Neurol and Psychol, 17, 1907, pp. 342-359. 



(No. 496 was issued on May 18) 



