These and many other difficulties encountered by the student of his own time are largely responsible for his opinions, often savouring as much of his idiosyncrasy, his professional and national prejudices as of an independent inquiry. In order to choose between the maze of highways and by-ways, in order to judge whether he moved forwards, backwards, or in a circle, he gropes for some kind of a compass and naturally clutches at that which his idiosyncrasy proffers. When we therefore meet with an appraiser of his own epoch, it behooves us to bear in mind the standpoint from which he has contemplated the world-phenomenon, and with what bias and prejudice his views have been coloured. The old Greek story of the sandal-maker who became prejudiced against a work of art because the artist had made a mistake in the arrangement of the sandal-strings, points its moral. The prejudices arising from trade, personal interests, and many other palpable sources are not difficult to trace and to evade, but where is the man whose views have not been influenced by his nationality, his religion, his favourite science or art, his love, his hatred, or his ambition? It is to such influences, often considered by the influenced as so many advantages and seldom sufficiently noticed by his critics, that we often owe the apparent profundity and exhaustiveness of an appreciation which in reality is one-sided. Education, and, still more, an intense study of one special branch of knowledge, rich in important and striking results, naturally tend to strengthen the student’s faith and his belief in the capabilities of his favourite science. The brain-cells, influenced by the will, and habitually becoming stimulated by presentations—emanating from the subject on which the student has concentrated his attention—adapt themselves gradually to the perception of such presentations, and by re-acting on other cells render the whole organism disposed to seek such presentations. In plain language, the specialist in one science has a great aptitude for discovering such causes and such effects as his favourite science has best elucidated, while he is tempted to overlook other causes and other effects which may be of equal or greater importance. The specialist attains to a mastery of his own subject, and often acquires a strong bias regarding other subjects, because he pursues his inquiries somewhat after the same fashion as the dog follows the scent of the game. By training, the dog is familiar with the smell of the animal pursued, and, bent on following the trail, he pays no attention to any other scents or smells that he encounters in his course. In the same way the specialist rapidly perceives and minutely studies any phenomena, however slight, with which his favourite science has rendered him familiar, while he is apt to disregard phenomena demanding fresh studies and threatening to be inexplicable by investigation confined to the lines which he prefers to follow.