It was the first time in his life that he had ever encountered outside of the Bible a mind of the highest order, or listened to it, as it delivered over to mankind the astounding treasures of its knowledge and wisdom in accents of appealing, almost plaintive modesty. That day the lad changed his teachers. Of the session more than two months yet remained. Every few days he might have been seen at the store, examining books, drawing money reluctantly from his pocket, hurrying away with another volume. Sometimes he would deliver to the clerk the title of a work written on a slip of paper: an unheard-of book; to be ordered--perhaps from the Old World. For one great book inevitably leads to another. They have their parentage, kinship, generations. They are watch-towers in sight of each other on the same human highway. They are strands in a single cable belting the globe. Link by link David's investigating hands were slipping eagerly along a mighty chain of truths, forged separately by the giants of his time and now welded together in the glowing thought of the world. Not all of these were scientific works. Some were works which followed in the wake of the new science, with rapid applications of its methods and results to other subjects, scarce conterminous or not even germane. For in the light of the great central idea of Evolution, all departments of human knowledge had to be reviewed, reconsidered, reconceived, rearranged, rewritten. Every foremost scholar of the world, kindling his own personal lamp at that central sunlike radiance, retired straightway into his laboratory of whatsoever kind and found it truly illuminated for the first time. His lamp seemed to be of two flames enwrapped as one; a baleful and a benign. Whenever it shone upon anything that was true, it made this stand out the more clear, valuable, resplendent. But wherever it uncovered the false, it darted thereat a swift tongue of flame, consuming without mercy the ancient rubbish of the mind. Vast purification of the world by the fire of truth! There have been such purifications before; but never perhaps in the history of the race was so much burned out of the intellectual path of man as during the latter half of the nineteenth century.