This paper deals with some problems arising from the controversy between the continuity and noncontinuity theories of discrimination learning. In the first place, it seems to the writer that these two opposed interpretations emphasize respectively the different aspects of learning. Experimental studies of stimulus generalization, presolution period and simultaneous or succesive presentation of stimuli have yielded that are not decisive with respect to the validity of either of the theories. Some are favorable to continuity theory and the others are to noncontinuity theory. It appears that these inconsistencies are due to the differences in the experimental operations and not to the nature of the learning itself, and further researches on this problem are urgently called for. Recent inteprretations of discrimination learning have pointed out that there are two different steps in the formation of learning process. They claim that the sensory-sensory conditioning (Maier and Schneirla), the conditionality (Mowrer), the discriminability (Lawrence), the sensory integrating process (Bitterman), the discriminative effect (Estes), etc. precede simple conditioning or response generalization. The two different steps are cognition and response. However, these interpretations invite a study of comparative psychology which includes not only mammals, but also other animals lower in the evolutionary scale. From this point of view, recent ethological studies of animal behavior are of great value to the understanding of discrimination learning.