this close association with memory (conscious or unconscious) that an odour is agreeable or disagreeable. In itself an odour is neither attractive nor repulsive. The acrid fumes of sulphur, chlorine, ammonia, and such bodies are not simply "odours" but corrosive chemical vapours, which act painfully upon the nerves of common sensation within the air-passages of the nose and throat and not exclusively, if at all, on the terminations of the olfactory nerves. An odour--that which acts on the special nerves of smell distributed in chambers of the nose--acquires its attractive or its repulsive quality only as the result of mental association with what is beneficial (suitable food, mates, friends, safety, home, the nest), or with what is injurious (unsuitable food, poison, enemies, danger, strange surroundings, solitude). Hence it is intelligible that the man accustomed to garlic or onions in his food is strongly attracted by their smell. So too the man whose tribe or companions have learnt by necessity to eat slightly putrid meat, fish, and cheese is attracted by their odour, though for others these odours are associated rather with what is poisonous and injurious. The dislike of the smell of sewer-gas and foul accumulations of refuse was not known to former generations of men (even in European cities a couple of hundred years ago) any more than it is to-day to the more unfortunate poorer classes, to many modern savages, to hyenas, and several other animals and birds which inhabit lairs and caves which they make foul. The odour of putrescence has become actually painful and almost intolerable to the more cleanly classes of mankind, owing to the association with it, as the result of education, of fear of disease and poisoning. Either conscious or unconscious association of an odour with what is held, either as the result of tradition or through personal experience, to be beneficial and of pleasant memory, or, on the contrary, injurious and of painful connection, determines man's liking for and choice or rejection of, odours and flavours. One can account with fair success on this basis for one's own preferences and dislikes in the matter.