Olfaction is generally felt to be the lowest, the most animal, of the senses, so much so that in polite society it is scarcely good manners to mention smells, and I am well aware of the risks I run in writing a book on the subject. And yet this feeling is by no means false modesty, because it is, first and foremost, to the animal in us that smell makes its appeal. None of the other senses brings so frankly to notice our kinship with the brute. Olfaction is, indeed, one of the primitive senses of animal life. And in man, as it happens, while vision has constructed for itself a highly complicated camera-like end-organ, and hearing has produced an apparatus even more elaborate, the olfactory organ, on the other hand, remains primitive, its essential structure having undergone no apparent evolutionary change from the simplest and earliest type. This, perhaps, is scarcely the proper way of expressing the situation. Evolutionary change has, as a matter of fact, occurred, but it reaches its highest development not in man, but in terrestrial mammals otherwise inferior to him—in the dog, for example. For once, man does not occupy the apex of the evolutionary pyramid. Olfactory development, high or low, is linked up with the natural habits of the different species. Thus, mammals which go about on all fours, whose visual outlook is restricted and whose muzzle is near the ground, are the most highly gifted; those, again, like the seals, porpoises, whales, and walruses, which have reverted from a terrestrial to an aqueous environment, where smell is of less value to them, show poorly developed olfactory organs; and finally, the apes and man, living habitually above the ground, the former in trees, the latter on his hind legs, and relying chiefly upon vision, also show a decline from the high point reached by four-footed mammalians. The animals of this kingdom are thus divided into macrosmatic and microsmatic groups. To the latter man belongs, but we must add that his olfactory sense has not yet degenerated so completely as that of certain other species (porpoises, etc.). It is, of course, common knowledge that in most of the animals we are closely acquainted with the sense of smell is infinitely more delicate and acute than ours, so much so, indeed, that the imagination can on occasion scarcely conceive theirs to be of the same nature. As a matter of fact, many authorities incline to the belief that not only mammalians and other vertebrates, but also insects, must be guided to their food and to their love-mates by some kind of perception, by some mysterious sense, of which we are totally devoid. As this is a division of our subject of the highest interest, and one to which we shall have occasion to recur at intervals throughout this treatise, we shall discuss the matter as fully as the space at our disposal will permit.