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....
[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.”
In criticism of this conclusion of Fabre, however, we must again draw attention to the fact that in the case of the Greater Peacock he found that a plug of cotton-wool was sufficient to prevent the emanation leaving the immediate neighbourhood of the female, a circumstance strongly in favour of some material exhalation which was caught and held by the cotton-wool filter. Again, in the case of the Banded Monk, the suggestion of odour is unmistakable in the tainting, as it were, of substances in her vicinity with her emanation. Further, if the guide to the males were something like a luminous undulation we should expect that, like the Bolboceros beetle and the badger, there would have been no blundering and going astray; they would have precipitated themselves straight on to the female, or as near to her as they could get.