If Horace Walpole, a contemporary of the eighteenth-century novel from its actual start to practically its finish, could give us thousands and all but tens of thousands of phrases that want but a little of being novel-conversation ready made, why could not the other people make it for their own purposes?
But we have got no answer to these questions: and probably there is none.
The way in which Scott and Miss Austen themselves simultaneously found out the secrets of the two kinds of novel is no doubt, as such ways always are, in the larger part mysterious: but to a certain extent it can be explained and analysed, independently of the direct literary genius of each.

One of the greatest gifts of Scott--one with which the non-historical novelist can dispense as little as his brother the historical--was that "genius of history" with which Lord Morley--a critic not likely to be misled by sympathy in some respects at any rate--has justly credited him.

For unless you have this "historic sense," as it has been more generally and perhaps better termed (though to the intense disgust of some professed historians), it is not only impossible for you to delineate scene and character at a distance from your time, but you become really disqualified for depicting your own time itself.

You fail to distinguish the temporary from the permanent; you achieve perhaps a fairly faithful copy of actual manners and fashions, but you do nothing more, and as the subject dies so does the picture.

Contrast Hook, say, with Thackeray, and the difference will emerge at once.
Secondly, Scott had, besides this historic sense and the relish for humanity which must accompany it, a knowledge of literature with which he has been too seldom credited to the full.