This perhaps may seem to be rather a fanciful suggestion for a scientifically trained writer to offer. But it is not wholly fanciful, since it has some support at least from theory (whatever that may be worth), and even from some considerations based upon solid fact. As to theory, we have already seen how Fabre arrived at the conclusion that the olfactory sense of certain insects is capable of receiving stimuli to which we are insensitive, stimuli which he surmised to be of the nature of an ethereal vibration. Consider too the following facts. It is well known that there are people who have an instinctive dislike of cats. The late Lord Roberts was one, and it is said of him that he was aware of the presence of his _bête noire_ before he caught sight of it. How was he made aware? The same instinctive aversion is felt by some people towards spiders. I myself know of one, a young girl, who cannot sleep if her bedroom contains one of these creatures. She, like Lord Roberts feels without knowing how when a spider is near her. Here also is a letter to a newspaper from a correspondent telling the same tale: “SIR, “I notice with interest that the official photographer who is to accompany Sir Ernest Shackleton’s _Quest_ expedition has an intense dislike of spiders. Can any of your readers explain this uncanny horror, which I believe is shared by a large number of people? “I myself loathe and fear spiders—so much so that I have been known on more than one occasion to go into a darkened room and to declare the presence of one of these creatures, my pet abomination being subsequently discovered.... “F. E.” What sense-organ—because there must be one—enables F. E. and others like him (or her) to detect the presence of a small creepy-crawly? We turn now to a series of medical cases which may throw some light upon this peculiarity. There are people who suffer from asthma when they go near horses. To enter a stable or to sit behind a horse is to them a certain means of bringing on an attack. This susceptibility and the peculiar form taken by the reaction remind us of hay fever. In sufferers from this troublesome complaint the pollen of certain plants has an irritating effect upon the mucous surfaces of the eyes, nose, and bronchial tubes. So in like manner recent investigation has shown that there is in the blood of the horse a proteid substance which acts as an irritant poison to those susceptible people. Their asthma, therefore, is merely a manifestation of the irritation produced by the poisonous body or its emanation when it is borne to them through the air. Similarly we are justified in arguing that cats and spiders may throw off an effluvium which is irritating to those susceptible to it.