Medical research bears so directly on the well-being of every one of us that it is astonishing that more people do not take an interest in it. The existing indifference may be attributed partly to the lack of good popular books describing recent advances in an easily comprehensible way, partly to the nomenclature adopted by the medical profession, which is apt to frighten the layman into imagining that the exotic polysyllables in question can be used only for phenomena of unimaginable complexity and obscurity. Complex they always are, of course, with the complexity of all natural manifestations, but very often the main lines of the problems which have arisen, and the methods of attack, can be stated in plain language. The book before us, which deals with the profoundly interesting subject of anaphylaxis, is not avowedly written for the enlightenment of the public, yet it is in most parts accessible to anybody with a slight knowledge of medicine, and, not being a "popular" book, it has the advantage of being free from the erroneous generalisations so often introduced in presenting a branch of science to the lay reader. The subject is described as one still in the course of development, and is not given that false air of completion so dear to the populariser. The phenomena of anaphylaxis are among the most striking in medical science, and have only recently been investigated, for although isolated cases of what we should now call anaphylaxis had been previously noted, Richet was the first to show the significance and extent of the subject in his memoir of 1902, in which he established many of the most important points. The essence of anaphylaxis is that the injection into an animal of a small quantity of one of certain substances--which in some cases are, and in others are not, poisons (with a poison the dose must, of course, be less than the fatal one)--puts the animal in a particular sensitive state, so that a very small second injection produces fatal results of a very violent and well-marked character. Some time must elapse after the first injection for the sensitive state to establish itself, and the second injection must be of exactly the same nature as the first, this latter fact constituting the so-called specificity of the anaphylactic effect. This specificity has been applied for identifying blood of a doubtful source, since an animal which has been sensitised with an injection of blood from a given species is sensitive only to blood of the same species. Further, the blood of an animal in the sensitive state can be used to render another animal sensitive, a result known as passive anaphylaxis.