Thus far, perhaps, the biological analogy may be pressed, if we remember that it is only an analogy. The evolution of a wagon or a battleship might offer an equally suggestive and an equally unsafe comparison. No one should be deceived by the analogy into thinking that what we call environment and heredity in literary species correspond in fact with their namesakes in the physical world. One play does not create another. It, along with countless other things, suggests ideas and impressions which are made into a play by the author. Each tragedy is the child of a mind, whose creative processes have little real resemblance to physical generation. To call the influence of "Hamlet" heredity, and the influence of the author's newspaper reading, or of his family, or his political beliefs, environment, is merely to assign arbitrary names. Again, art, unlike nature, is careless of the type and careful of the individual. A single play may live longer and have greater generating power than a whole species. "Othello" during the eighteenth and nineteenth centuries has, perhaps, had more influence upon English tragedy than all non-Shakespearean tragedies together. Sophocles is still germinating. It is as if we now had the venerable chief of the mastodons, surviving many of the species he had originated, and still creating his offspring to confuse the evolutions of the many other species he had already aided in forming. In literature we are attempting to trace the development of species, of which individuals live forever; to discriminate the agents in a complex creative process which we do not at all understand; to call one play the child of another when it is more truly the kaleidoscopic aggregate of much reading, much observation, much experience shaken into a new form by the author's creative imagination. Literary criticism may borrow from the natural sciences the evolutionary conception and some of its accompaniments; in particular, the demarcation of literary forms by the persistence of certain characteristics through changing conditions of nation or period, and the recognition of imitation as an important element in the creative process. It would seem, however, that further progress in the classification and explanation of literary phenomena is not to be gained by searching for additional analogies, but in the study and analysis of the phenomena themselves in the effort to discover the principles and laws of the mental processes peculiar to literature.