Learning by experience occurs when traces, records, or effects of a previous reaction are more or less irreversible and persist after the external factor determining them has ceased to act and so become factors in altering the course of subsequent reactions.

The biologist ordinarily thinks of development as something very different from such modification of behavior by experience, but from time to time the idea that the basis of heredity and development is fundamentally similar to memory has been advanced. More than forty years ago Hering . . . advanced this idea in general form. . . . Viewed in this way the whole course of development is a process of physiological learning, beginning with the simple experience of differential exposure to an external factor, and undergoing one modification after another, as new experiences in the life of the organism or of its parts in relation to each other occur. Memory and learning in the narrower, psychological sense represent that part of the general developmental learning process which concerns the minute pattern of certain regions of the nervous system in advanced stages of development, particularly in the higher animals. There is no evidence of any fundamental physiological difference between the general protoplasmic memory . . . and the higher forms of memory characteristic of the central nervous system.¹

There is therefore in the long run no unlearned behavior. Instinctive behavior is learned by the long-time exposure of the species to excitants; it is phylogenetically learned.

¹Child, C.M.n/an/an/an/a, , 248–249 (Henry Holt & Company. By permission).