BEHAVIOR OF VERTEBRATES 461 



remained there as was their custom during the regulai routine 

 of experimentation. A legular set of tests was then initiated. 

 Hunter was able to obtain several performances of a similar kind. 



HABIT FORMATION 



Amphibia. In connection with his bleeding experiments upon 

 the Mexican axolotl (Ambly stoma tigrinum) Haecker (8) has 

 made a study of learning and retention in relation to sexual 

 and other changes in the individual. The animals were trained 

 to distinguish between pieces of meat and wood of the same 

 size, drawn before them slowly in the jaws of a long pair of 

 forceps. The receptors involved in the discrimination were not 

 very thoroughly analyzed, the author resting content with the 

 statement that vision plays a very slight role, while a "special 

 function of the oral sense" and apparently the perception of 

 water currents are the chief factors in the reaction. The ques- 

 tion of the impulse to the reaction, hunger, has, on the contrary, 

 been treated at great length. Considerable variation in the 

 degree of hunger was found from day to day, seemingly associated 

 to some extent with atmospheric conditions, but it was found 

 that after the animals had been trained for a month the errors 

 made had little relation to the degree of hunger. 



The animals learned readily to distinguish between the meat 

 and the wood and to avoid the latter. The most noteworthy 

 thing about the learning curves is the ihythmic appearance of 

 periods in which numerous errors were made long after perfect 

 learning had been reached. Thus, in the case of a black female 

 which was studied for two and a half years, a large number of 

 errors were made in June, r9io, October, 1910, December, 1910, 

 May, 191 1, and the winter of 191 r, while the intervening periods 

 were almost without error. 



Averaging his results, the author finds that the periods of 

 error are distributed about two maxima, one in summer, one in 

 winter. Once the animals had learned to distinguish between 

 the meat and the wood the errors were not increased by rather 

 long interruptions of training or by fluctuations in the degree 

 of hunger. The two maxima agree with the periods of greatest 

 sexual activity and the author believes them to be the result 

 of the latter; that the physiological changes occurring during 



