The study of unilateral and reciprocal literary interactions, whether between individuals or nations, has been the oldest preoccupation of comparative literature. It is almost unbelievable, then, that neither the different forms of such interaction nor the methods of comparison between authors, groups, or national literatures, have generated any theory of how and why this contact occurs. A treasure-house of concrete data may be hidden in the many thousands of volumes of comparative literary studies published thus far; nevertheless, until now no one has undertaken the task of deriving generalizations from these data in order to build a theory of literary interaction. [...]