42 ROBERT M. YERKES 



the female and lost almost entirely his timidity in the apparatus. 

 She, however, continued to be rather timid throughout the sev- 

 eral months of work, although she was perfectly tame. The 

 differences in the nature of the reactions, as recorded in the 

 experimenter's record-book, can be appreciated only in the light 

 of these temperamental facts. 



The sex contrasts indicated in the above paragraphs one dare 

 not emphasize very strongly on the basis of observations on 

 two individuals, but they at least suggest the desirability of 

 further study of the sexes. It is the writer's opinion that they 

 agree sufficiently closely with the results obtained in the case 

 of other animals to justify their provisional acceptance. 



As has been repeatedly noted with other animals, there are 

 good and bad days in experimental work with ring-doves, — days 

 which are good or bad, not, so far as one may tell, because of 

 variation in the experimenter or his manipulation of the appa- 

 ratus, but chiefly because of variations in the condition of the 

 subjects. The experiments described in this paper were made 

 at about the same hour each morning, and it was quite impos- 

 sible for the experimenter to predict the outcome of a series 

 in the light of previous series, for the attention of the doves to 

 the situation seemed to vary independently of any conditions 

 or group of conditions which the experimenter could take into 

 account. There are animals which can be relied upon to work 

 steadily and fairly predictably. The ring-dove is not one of them. 



The writer has been led to reflect, because of the outcome of 

 this series of observations, on the possible relation of the sim- 

 plicity of the experimental situation to the results. He was 

 compelled to devote several weeks to the establishment of a 

 simple habit in two ring-doves, a habit which was next to value- 

 less except as a preparation for further observations. It is 

 natural that during this long period of preparation he should 

 frequently wonder whether the desired end might not be gained 

 more quickly by a different method. It seems probable that a 

 complex situation would have proved more favorable, and that 

 had the two stimuli varied in other respects than in intensity, 

 the animal's attention would more readily have been directed to 

 them and more steadily held upon them. The matter is men- 

 tioned here because it is obviously of extreme importance to 



