40 ROBERT M. YERKES 



Instead, they seem to indicate that for the female dove, the 

 red was so dark that it tended to be confused with the black, 

 or at least was not accepted as the equivalent of the light area 

 which the bird had previously learned to choose. 



In this red-black training, it was possible to give each dove 

 twenty trials in succession. As a result of one hundred and 

 forty trials, number 3 was reacting properly ninety per cent of 

 the time. Curiously enough, the male, number 4, chose the red 

 eighteen times out of twenty in his first series, and showed 

 throughout his reactions, in the red-black training, ability to 

 respond to these two stimuli much as he had to the light and 

 dark achromatic stimuli. This is, of course, wholly in agree- 

 ment with the results of the preference tests, which clearly indi- 

 cated that the red stimulus for some reason possessed a higher 

 stimulating value for the male than for the female. 



It is, of course, impossible to say, on the basis of the red- 

 black results, that either bird responded to the chromatic differ- 

 ence instead of to the intensity difference of the stimuli. It is 

 doubtless safer to assume that the latter alone was the basis 

 of choice. 



Beginning on May 9th both doves were presented with the 

 red and green stimuli which on April 21st had been offered as 

 a basis for preference reactions, with the difference that now 

 they were required to choose the red and to avoid the green on 

 penalty of electric stimulation. Again, each daily series con- 

 sisted of twenty successive trials. The female exhibited, at 

 first, slight ability to distinguish the two stimuli and to respond 

 appropriately, but after three hundred and eighty trials, she 

 was reacting perfectly. The male, on the contrary, reacted 

 perfectly even from the first, his second series of twenty trials 

 including no mistakes. It is thus fairly clear that he responded 

 to the intensity difference of the two chromatic stimuli, and it 

 seems wholly probable, in view of the gradual development of 

 the habit, that she also acquired the ability to respond to the 

 same difference. 



From these preliminary observations, it seems safe to con- 

 clude that for the ring-dove a red and a green from the spec- 

 trum of the carbon arc, of the wave lengths designated above, 

 and of approximately the same energy, as measured by the 

 selenium cell, are sufficiently unlike in stimulating value to be 



