Philological and textual criticism: the study of literature as an aid to tracing the origins of words, the evolution of languages, and the establishment of accurate texts of authors' works, rather than reading the literature as a subject of study which had its own, non-linguistic interests. The philological branch of linguistics originated in Germany during the nineteenth century, and it rose to prominence in Anglo-American colleges established on the German university model (e.g., University of Chicago and Johns Hopkins), which founded university presses so that their doctoral students' dissertations could be published to the rest of the academic world. A philological critic would study the patterns of word use in Chaucer, perhaps to trace the gradual erosion of the sounded final "-e" in Middle English, rather than reading the "Canterbury Tales" as a sociopolitical artifact, an aesthetic object, etc. This kind of philological evidence was crucial to "textual critics" when they turned to establishing the "canon" of texts which could be accurately attributed to any single author.
Early printers tended to publish the best manuscripts they could find, but if the manuscripts contained works by various authors, the printer might attribute all of them to the most famous or politically acceptable author among them. In this way, works by several lesser authors became mixed up with Chaucer's works, and all of Sir Thomas Wyatt's poetry first circulated in "Tottel's Miscellany" (1557) under the name of Henry Howard, the earl of Surrey, because Wyatt's name had been made politically unacceptable because of his son's 1554 rebellion. Plays were especially vulnerable to piratical practices of early printers before the invention of authors' copyright. Many of Shakespeare's plays first were published without his permission in cheap, usually inaccurate "quarto" (quarter-page-sized) editions by printers working with actors or short-hand specialists who copied the dialogue as well as they could during performances. After the publication of the "First Folio" (1623), a large-format, deluxe edition printed by a group of the author's friends after his death, textual critics have been working for centuries to establish the best possible text of these plays. As recently as the 1990s, neither the folio nor the quarto text of King Lear were found to be adequate on their own because each contained passages that philological analysis could demonstrate to be Shakespeare's work, and that were too important to the play's dramatic structure to be omitted. As a result, the Norton Anthology of English Literature published a "composite" edition, taking the best parts of both, but possibly presenting us with a play that Shakespeare's audiences never saw in this precise form.
In addition to cleaning up authors' canonical works, the philological critics' greatest contribution to the study of English literature was the restoration of Middle English and Old English as readable precursors of Modern English, giving us a more firm grasp of the sound and sense of nearly a thousand years of literature. The other titanic achievements they have left us are the Oxford English Dictionary ("OED") and the Middle English Dictionary ("MED"), which were produced by thousands of scholars who sent 3x5 cards containing what they believed to be early uses of English words to a central clearing house in England (now stored at University of Michigan, Ann Arbor), where painstaking work sorted each word's linguistic history, based on its first recorded appearance in published or surviving manuscript works. The library has a subscription to the OED which you can access from its "Databases A-Z" page, and you should use it regularly, and reverently, if for no other reason than to honor the amazing discipline and energy of the scholars who built it.