It was agreed between us that the dog should act according to his own instincts, receiving the customary reward, after each discovery, no matter what its size, of a crust of bread the size of a finger-nail.  Every spot scratched by his paw should be excavated, and the object indicated was to be extracted without reference to its marketable value.  In no case was the experience of the master to intervene in order to divert the dog from a spot where the general aspect of things indicated that no commercial results need be expected, for I was more concerned with the miserable specimens unfit for the market than with the choice specimens, though of course the latter were welcomed.

Thus conducted, this subterranean botanising was extremely fruitful.  With that perspicacious nose of his the dog obtained for me both large and small, fresh and putrid, odorous and inodorous, fragrant and offensive.  I was amazed at my collection, which comprised the greater number of the hypogenous fungi of the neighbourhood.

What a variety of structure, and above all of odour, the primordial quality in this question of scent!  There were some that had no appreciable scent beyond a vague fungoid flavour, more or less common to all.  Others smelt of turnips, of sour cabbage; some were fetid, sufficiently so to make the house of the collector noisome.  Only the true truffle possessed the aroma dear to epicures.  If odour, as we understand it, is the dog’s only guide, how does he manage to follow that guide amidst all these totally different odours?  Is he warned of the contents of the subsoil by a general emanation, by that fungoid effluvium common to all the species?  Thus a somewhat embarrassing question arises.

I paid special attention to the ordinary toadstools and mushrooms, which announced their near advent by cracking the surface of the soil.  Now these points, where my eyes divined the cryptogam pushing back the soil with its button-like heads, these points, where the ordinary fungoid odour was certainly very pronounced, were never selected by the dog.  He passed them disdainfully, without a sniff, without a stroke of the paw.  Yet the fungi were underground, and their odour was similar to that I have already referred to.

I came back from my outings with the conviction that the truffle-finding nose has some better guide than odour such as we with our sense-organs conceive it.  It must perceive effluvia of another order as well; entirely mysterious to us, and therefore not utilised.  Light has its dark rays—­rays without effect upon our retinas, but not apparently on all.  Why should not the domain of smell have its secret emanations, unknown to our senses and perceptible to a different sense-organ?

If the scent of the dog leaves us perplexed in the sense that we cannot possibly say precisely, cannot even suspect what it is that the dog perceives, at least it is clear that it would be erroneous to refer everything to human standards.  The world of sensations is far larger than the limits of our own sensibility.  What numbers of facts relating to the interplay of natural forces must escape us for want of sufficiently sensitive organs!