Literature comes more and more to mean imaginative literature or writing about imaginative literature.

The mass of writing comes to be taken not as literature, but as argument or information; we consider it purely from the point of view of its subject matter.

A comparison will make this at once clear.

When a man reads Bacon, he commonly regards himself as engaged in the study of English literature; when he reads Darwin he is occupied in the study of natural science.

A reader of Bacon's time would have looked on him as we look on Darwin now.
The distinction is obviously illogical, but a writer on English literature within brief limits is forced to bow to it if he wishes his book to avoid the dreariness of a summary, and he can plead in extenuation the increased literary output of the later age, and the incompleteness with which time so far has done its work in sifting the memorable from the forgettable, the ephemeral from what is going to last.