The student of culture is concerned with a characteristic which man displays more markedly than any other known creature — the ability to transmit what he has learned. In following the procedure I suggest, the learning of Homo sapiens would be treated as a further specialization of the concept of grades, with the recognition that in some species — possibly even in some orders — the ability to learn may represent not only an improvement, in an evolutionary sense, but also an increase in vulnerability. Man's unique, high ability to learn, coupled as it is with a small amount of built-in behavior, represents such a vulnerability.

p. 30-31 - Continuities in Cultural Evolution (1964)