Is this sense, then, as we see it in operation in the badger, in the truffle-hunting dog, in the Bolboceros beetle, and still more wonderfully in the Peacock and Banded Monk moths, drawn to their mates “from the edge of the horizon,” and, it may be, against the wind—is this sense the same as our own sense of olfaction, only much more acute? Fabre finds some difficulty in believing that it can really be the same. “Odour,” he argues, “is molecular diffusion.” But nothing material, nothing our senses can perceive, is emitted by these moths, and yet they can summon their mates from relatively enormous distances. However fine may be the divisibility of matter, Fabre’s mind refuses to entertain the suggestion that this far-flung summons is addressed to a sense of smell of the same nature as ours. It would be tantamount, he says, “to reddening a lake with an atom of carmine, to filling immensity with nothing.” It is impossible not to sympathise with this opinion, but caution compels us to say that for the most striking of these observations, that of the calling of the males against a high wind, we should like to have confirmation by some independent observer. Besides, I think perhaps Fabre would have hesitated to express his scepticism regarding the power of insect olfaction had he known more of the marvels of the human sense. Vanillin, for example, is perceptible by us as a smell when it amounts to no more than 0·000000005 gram in a litre of air; and we can perceive mercaptan, a substance with a garlicky odour, in a dilution of 1/460,000,000 of a milligram in fifty cubic centimetres of air (approximately 0·0000000026 of a grain in a little over three cubic inches of air!) (See also p. 108.) What is this but immensity filled with nothing? And yet we, even we, microsmatic though we are, can perceive that “nothing.” But we must pick up again the thread of Fabre’s argument. Baffled as he feels himself to be when he regards olfaction in the light of these observations of his, he goes on: “For emission substitute undulation, and the problem of the Great Peacock is explained. Without losing any of its substance a luminous point shakes the ether with its vibrations and fills a circle[1] of indefinite width with light.... Footnote 1: A sphere rather. “It does not emit molecules; it vibrates; it sets in motion waves capable of spreading to distances incompatible with a real diffusion of matter. “In its entirety smell would thus seem to have two domains: that of particles dissolved in the air and that of ethereal waves. The first alone is known to us.... “The second, which is far superior in its range through space, escapes us altogether, because we lack the necessary sensory equipment. The Great Peacock and the Banded Monk know it at the time of the nuptial rejoicings. And many others must share it in various degrees according to the exigencies of their mode of life.”