So marked is the integrity and continuity of the type that tragedy lends itself, better perhaps than most other forms, to the biological analogy. The processes which we have been tracing are evolutionary. Whether we consider the main type or its varying forms, we are reminded constantly of the laws governing the origin and development of natural species. The history of the Elizabethan drama in particular affords an example of the origin, development, culmination, and degeneration of a literary species, which might be analyzed closely as Brunetière has analyzed French tragedy of the seventeenth century. Created from a cross-fertilization of Seneca on the medieval drama, it appears in dubious forms of morality and chronicle, springs into full integrity in Marlowe, reaches its culmination in Shakespeare, and degenerates under the changed environment of the social and theatrical conditions that followed the death of Elizabeth. But the analogy is not less applicable to the whole history of tragedy. The slow development of variations and new species under changing environment is found in every period, as in the formation and growth of the revenge play or in the development of the sentimental tragedy of the eighteenth century. The quick formation of species by mutation also has its parallels, as in the sudden appearance of Marlowe or of the heroic tragedy bred from the Beaumont-Fletcher play and French romance. In the persistence of the stage villain through all forms and periods, we might even discover one of Mendel's unit characters. The reversion to an earlier form appears in the return of Lee or of the Romanticists to the Elizabethans. And the tendency of individual plays to regress to the main type has been a constant and on the whole perhaps the most potent force of the development.