The energy and enthusiasm of these writers, and the good work they may be doing in their own science, render them more or less immune from attack; but every now and then it is necessary to say clearly that such extravagant generalisations profane the modesty of science: whose heritage it is to recognise the limitations of partial knowledge, and to be always ready to gain fresh experience and learn about the unknown. The new and unfamiliar is the vantage ground, not of scientific dogmatism, but of scientific inquiry. * * * * * The expository or theoretical part of this book may at first appear too abstract for the general reader who has had no experience of the kind of facts already described. Such reader may fail to see a connexion between this more didactic portion and the illustrations or examples which have preceded it; but if he will give sufficient time and thought to the subject, the connexion will dawn upon him with considerable vividness. It has always seemed to the author legitimate, and in every way desirable, for an experimenter to interpret and make himself responsible for an explanation or theory of his observations, so far as he can. To record bare facts and expect a reader of the record to arrive at the same conclusion as that reached by one who has been immersed in them for a long time, is to expect too strenuous an effort, and is not a fair procedure. Such a practice, though not unusual and sometimes even commended in physical science, is not followed by the most famous workers; and it has been known to retard progress for a considerable time by loading the student with an accumulation of undigested facts. The hypothesis on which an observer has been working, or which he has arrived at in the course of his investigations, may or may not be of permanent value, but if his experience has led him to regard it as the best solution so far attainable, and if he is known not to be a specially obstinate or self-opinionated person, his views for what they are worth should be set forth for the guidance of future inquirers. If he mauls the facts in his direction, he will be detected; but such an accusation is a serious one, and should not be made lightly or without opportunity for reply. The string on which beads are strung may not be extremely durable, and in time it may give place to something stronger, but it is better than a random heap of beads not threaded on anything at all.