of which nothing at all is left, not even the threshold.

If any doubts remained as to the incapacity of the Mason-bee of the
Walls to know her nest other than by the place which the pebble
occupies on the ground, here is something to remove them: for the nest
of one Mason-bee, I substitute that of another, resembling it as
closely as possible in respect to both masonry and storage. This
exchange and those of which I shall speak presently are of course made
in the owner's absence. The Bee settles without hesitation in this
nest which is not hers, but which stands where the other did. If she
was building, I offer her a cell in process of building. She continues
the masonry with the same care and the same zeal as if the work
already done were her own work. If she was fetching honey and pollen,
I offer her a partly-provisioned cell. She continues her journeys,
with honey in her crop and pollen under her belly, to finish filling
another's warehouse. The Bee, therefore, does not suspect the
exchange; she does not distinguish between what is her property and
what is not; she imagines that she is still working at the cell which
is really hers.

After leaving her for a time in possession of the strange nest, I give
her back her own. This fresh change passes unperceived by the Bee: the
work is continued in the cell restored to her at the point which it
had reached in the substituted cell. I once more replace it by the
strange nest; and again the insect persists in continuing its labour.
By thus constantly interchanging the strange nest and the proper nest,
without altering the actual site, I thoroughly convinced myself of the
Bee's inability to discriminate between what is her work and what is
not. Whether the cell belong to her or to another, she labours at it
with equal zest, so long as the basis of the edifice, the pebble,