In returning to the dovecote and the burrow, when man has artificially made them lose their bearings and carried them to great distances, in unfamiliar directions and into regions which they have not yet visited, are the Pigeon and the Cerceris guided by recollection? Is memory their compass when, on reaching a certain height, whence they can, so to speak, pick up the scent after a fashion, they dart with all their power of wing towards the horizon where their nests are? Is it memory that traces their road through the air, across regions which they are seeing for the first time? Obviously not: there can be no recollection of the unknown. The Wasp and the bird are unacquainted with the country around; nothing can have told them the general direction in which they were moved, for the journey was made in the darkness of a closed basket or a box. Locality, relative position: everything is unknown to them; and yet they find their way. They therefore have something better than mere memory as a guide: they have a special faculty, a sort of topographical sense of which we cannot possibly form an idea, having nothing similar ourselves. I will show by experiment how subtle and precise this faculty is within its narrow province, and also how obtuse and dull it becomes when driven to depart from the usual conditions in which it acts. This is the invariable antithesis of instinct. A Bembex, actively engaged in feeding her larva, leaves the burrow. She will return presently with the produce of the chase. The entrance is carefully stopped up with sand, which the insect has swept there backwards before going away; there is nothing to distinguish it from other points of the sandy surface; but this does not trouble the Wasp, who finds her door with a skill which I have already emphasized. Let us devise some insidious plot and change the conditions of the locality in order to perplex the insect. I cover the entrance with a flat stone, the size of my hand. The Wasp soon arrives. The great change effected on her threshold during her absence appears to cause her not the slightest hesitation; at least, the Bembex at once alights upon the stone and tries, for an instant, to dig into it, not at random but at a spot corresponding with the opening of the burrow. The hardness of the obstacle soon dissuades her from her enterprise. She then runs about the stone in every direction, goes all round it, slips underneath and begins to dig in the exact direction of her dwelling.