We may find the nature of the literary species determined by constant principles corresponding to environment and heredity in the evolution of natural species. Environment as a factor in literature has long been recognized by criticism, and has been apparent in every play that we have examined. Each period has been distinguished by theatrical, social, and literary conditions peculiar to itself and constituting the change-producing environment of the drama. Tragedy has at every stage responded to these changing conditions. And the law of heredity is also paralleled. No play has been without its inheritance. The most original, as Shakespeare's "Hamlet," Otway's "Orphan," Lillo's "Barnwell," and Shelley's "Cenci," have shown their indebtedness no less clearly, if less slavishly, than the more commonplace individuals. The classical tradition transformed the English breed as the Arabian stock has the racing-horse; the French influence changed the very anatomy of the species. Our study must surely have called attention to the extraordinary force that imitation has exercised in the creation of tragedy. It seems, indeed, the generating power. Men are forever imitating, but they cannot imitate without change. In these changes, the variations due to environment--personal, theatrical, literary, social--arise the individual peculiarities, the beginnings of new species, the element of growth. The great mass of tragedies, however, differentiate themselves only feebly or slightly from the type. They are imitations that preserve all the essential characteristics of their originals. Some ideas, some plays, some traditions, have an astonishing fecundity; other stocks, procreative for a while, soon turn barren. But, destroy the faculty of imitation, and the generation of literary forms would seem wellnigh impossible.