We must not, however, confine attention to the higher multicellular animals and plants. One of the most important factors in the environment is the existence of bacteria; and it is of great importance that an outline of bacteriology should be included in our course of Common-sense Biology. This outline should be kept close to the common necessities of everyday life. For the sake of making clear and real to the mind the manner in which bacteria multiply and the extreme rapidity of the process, a certain amount of microscopical work ought to be done, the examples being few, but carefully chosen. This kind of work, nevertheless, should be kept subordinate. The effects wrought by bacteria in water, earth and air, in stored foodstuffs, and in the tissues of the living body are the important subjects for study; and naturally, connected with these, the conditions which permit the access of bacteria or which, in the case of noxious bacteria, will best ensure protection. The rationale of toxins and anti-toxins, with the relations of these to the blood-serum should also, in a general way, be known; and moreover the student should be prepared to learn that many diseases, which are at present very imperfectly understood--we may take, for instance, forms of insanity--have their _vera causa_ in the action of toxins, and require to be treated accordingly. Perhaps, for those who cannot take more than the shorter courses of our Common-sense Biology, it will be sufficient to consider only those forms of inimical bacteria which we have to combat in our own islands. But the writer would strongly urge that, at least among women of the leisured classes, this instruction should be extended to cover the bacterial and other minute parasitic forms of disease most prevalent in our colonies and in our foreign possessions. The wives of officers, civil servants and missionaries ought to know, in a clear, scientific way, the causes, modes of attack, and methods of prevention of the principal tropical diseases, so far as these have at present been made out. METHOD OF STUDY What should be the method of this study?