The Hydnocystis, the food of the Bolboceras, emits no such brutal emanations as these, which readily diffuse themselves through space; it is inodorous, at least to our senses.  The insect which seeks it does not come from a distance; it inhabits the places wherein the cryptogam is found.  Faint as are the effluvia of this subterranean fungus, the prospecting epicure, being specially equipped, perceives them with the greatest ease; but then he operates at close range, from the surface of the soil.  The truffle-dog is in the same case; he searches with his nose to the ground.  The true truffle, however, the essential object of his search, possesses a fairly vivid odour.

But what are we to say of the Great Peacock moth and the Oak Eggar, both of which find their captive female?  They come from the confines of the horizon.  What do they perceive at that distance?  Is it really an odour such as we perceive and understand?  I cannot bring myself to believe it.

The dog finds the truffle by smelling the earth quite close to the tuber; but he finds his master at great distances by following his footsteps, which he recognises by their scent.  Yet can he find the truffle at a hundred yards? or his master, in the complete absence of a trail?  No.  With all his fineness of scent, the dog is incapable of such feats as are realised by the moth, which is embarrassed neither by distance nor the absence of a trail.

It is admitted that odour, such as affects our olfactory sense, consists of molecules emanating from the body whose odour is perceived.  The odorous material becomes diffused through the air to which it communicates its agreeable or disagreeable aroma.  Odour and taste are to a certain extent the same; in both there is contact between the material particles causing the impression and the sensitive papillae affected by the impression.

That the Serpent Arum should elaborate a powerful essence which impregnates the atmosphere and makes it noisome is perfectly simple and comprehensible.  Thus the Dermestes and Saprinidae, those lovers of corpse-like odours, are warned by molecular diffusion.  In the same way the putrid frog emits and disseminates around it atoms of putrescence which travel to a considerable distance and so attract and delight the Necrophorus, the carrion-beetle.

But in the case of the Great Peacock or the Oak Eggar, what molecules are actually disengaged?  None, according to our sense of smell.  And yet this lure, to which the males hasten so speedily, must saturate with its molecules an enormous hemisphere of air—­a hemisphere some miles in diameter!  What the atrocious fetor of the Arum cannot do the absence of odour accomplishes!  However divisible matter may be, the mind refuses such conclusions.  It would be to redden a lake with a grain of carmine; to fill space with a mere nothing.

Moreover, where my laboratory was previously saturated with powerful odours which should have overcome and annihilated any particularly delicate effluvium, the male moths arrived without the least indication of confusion or delay.