86 CORA D. REEVES 



ination occurs at matched brightness for the fish, it may 

 be taken as evidence of color vision. This general form 

 of experiment was used by Frisch (1911) in the experi- 

 ments in which the fish discriminated a colored food tube 

 in a series of grays or colored tubes. The results were 

 rightly judged to be an indication of color vision. Had 

 discrimination not taken place in this case, the result 

 would ordinarily have been taken to indicate a lack of 

 color vision. This conclusion could be accepted only in 

 case the trials were long enough continued at matched 

 brightness of the two colors to make it certain that failure 

 to discriminate persisted. In other words, in such a series 

 temporary failure to discriminate at matched brightness 

 is not conclusive. This source of error has not "appeared 

 in previous work with fish. A reference to the graphs 

 (fig. 13) shows how it might appear. If in the case of Small 

 Sunfish, upon failure to discriminate at slit- width 0.9 mm., 

 (matched brightness) the slit width had been at once con- 

 siderably reduced, discrimination would have been re- 

 established. ■ The curve might then have been interpreted 

 as evidence of brightness discrimination with failure at 

 matched brightness. Only by continuing the trials at matched 

 brightness and securing return of discrimination at this slit 

 width does it give evidence of discrimination of wave- 

 lengths. 



e. Error may arise from exclusive use of the method of 

 response. — This method does not accurately test the pDtenti- 

 ality for discrimination for fish. It shows not inability but 

 merely failure to respond under the conditions of the ex- 

 periment. Only a method which necessitates the formation 

 of a definite habit of response to a definite sensory stimulus 

 can be expected to show the ability of the fish to respond. 



C. General Considerations 



a. Difference in species of fish investigated. — All fish 

 that have been used in investigations on color vision belong 

 to the group Pisces, one of the three classes of vertebrates 

 popularly referred to as fish. Few species have been in- 

 vestigated. It has been pointed out that the capacity to 

 discriminate by means of wave-lengths is not the same in 



