"Treasure Island" is popular because it stirs and satisfies two instinctive cravings of mankind, the love of romantic adventure, and the desire for sudden wealth. This is not true, or rather it is not the whole, or even the important, truth, in "Main Street." There the chief appeal is to an idea not an instinct. We left the war nationally self-conscious as perhaps never before, acutely conscious of the contrasts between our habits, our thinking, our pleasures, our beliefs, and those of Europe. When the soldiers oversea talked generalities at all it was usually of such topics. The millions that never went abroad were plucked from their Main Streets, and herded through great cities to the mingled companionship of the camps. "Main Street," when it came to be written, found an awakened consciousness of provincialism, and a detached view of the home town such as had never before been shared by many. Seeing home from without was so general as to constitute, not a mere experience, but a mass emotion. And upon this new conception, this prejudice against every man's Main Street, the book grasped, and thrived. In like manner, "Uncle Tom's Cabin" grew great upon its conception of slavery. "Robert Elsmere" swept the country because of its exploitation of freedom in religious thought. No one of these books could have been written, or would have been popular if they had been written, before their precise era; no one is likely long to survive it, except as a social document which scholars will read and historians quote. Roughly then, the appeal which makes for popularity is either to the instinctive emotions permanent in all humanity, though changing shape with circumstances, or to the fixed ideas of the period, which may often and justly be called prejudice. A book may gain its popularity either way, but the results of the first are more likely to be enduring. "Paradise Lost," the least popular of popular poems, still stirs the instinctive craving for heroic revolt, and lives for that quite as much as for the splendors of its verse. Dryden's "Hind and the Panther," which exploited the prejudices of its times, and was popular then, is almost dead. What are these instinctive cravings that seek satisfaction in fiction and, finding it, make both great and little books popular? Let me list a few without attempting to be complete.