If smell, as we understand it, is the Dog’s only guide, how does he manage to find his way through all these incongruous odours? Is he apprised of the contents of the soil by a general emanation, the fungous effluvium common to the different species? In that case an extremely embarrassing question arises. I paid some attention to the ordinary mushrooms, many of which, as yet invisible, announced their coming as imminent by cracking the surface of the ground. Now I never saw the Dog stop at any of those points where my eyes divined the cryptogam pushing back the earth with the thrust of its cap, points where the ordinary fungous smell was certainly most pronounced. He passed them by scornfully, with not a sniff, with not a stroke of his paw. And yet the thing was underground; and its reek was similar to others which he sometimes pointed out to us. I came back from the Dog’s school with the conviction that the truffle-detecting nose has a better guide than smell, in the sense in which our olfactory powers realize it. It must perceive, in addition, effluvia of a different order, full of mystery to us, who are not equipped accordingly. Light has its dark rays, which are without effect upon our retinæ, but not apparently upon all. Why should not the domain of smell have its secret emanations, unknown to our senses but perceptible to a differently constructed organ of smell? If the scent of the Dog leaves us perplexed to this extent, that it is impossible for us to say exactly or even to suspect what it perceives, it at least tells us plainly that we should be greatly mistaken to compare everything by human standards. The world of sensations is far larger than the limits of our sensibility admit. What a number of facts in the working of the forces of nature escape us for want of organs delicate enough to perceive them! The unknown, that inexhaustible field which the future will cultivate, holds harvests in store for us beside which our present knowledge is but a pitiful gleaning. Under the sickle of science sheaves will one day fall whose grain to-day would seem a senseless paradox. Scientific illusions? Not so, if you please, but undeniable and positive realities, affirmed by the animal world, which in certain respects has a great advantage over the world of man. In spite of his long professional practice, in spite of the aroma of the tuber which he is seeking, the rabassier cannot guess the presence of the truffle, which ripens in winter underground, at a depth of eighteen inches or so; he needs the aid of the Dog or the Pig, whose scent pries into the secrets of the soil. Well, these secrets are known to different insects even better than to our two helpers. In order to discover the tuber on which their family of grubs is to be fed, they possess a scent of exceptional perfection.