of a town to the other, threading his way through a labyrinth of
streets and alleys which he sees for the first time. Nor is it sight
that guides my Mason-bees, especially when they are let loose in the
thick of a wood. Their low flight, eight or nine feet above the
ground, does not allow them to take a panoramic view nor to gather the
lie of the land. What need have they of topography? Their hesitation
is short-lived: after describing a few narrow circles around the
experimenter, they start in the direction of the nest, despite the
cover of the forest, despite the screen of a tall chain of hills which
they cross by mounting the slope at no great height from the ground.
Sight enables them to avoid obstacles, without giving them a general
idea of their road. Nor has meteorology aught to do with the case: the
climate has not varied in those few miles of transit. My Mason-bees
have not learnt from any experience of heat, cold, dryness and damp:
an existence of a few weeks' duration does not allow of this. And,
even if they knew all about the four cardinal points, there is no
difference in climate between the spot where their nest lies and the
spot at which they are released; so that does not help them to settle
the direction in which they are to travel.

To explain these many mysteries, we are driven therefore to appeal to
yet another mystery, that is to say, a special sense denied to
mankind. Charles Darwin, whose weighty authority no one will gainsay,
arrives at the same conclusion. To ask if the animal be not impressed
by the terrestrial currents, to enquire if it be not influenced by the
close proximity of a magnetic needle: what is this but the recognition
of a magnetic sense? Do we possess a similar faculty? I am speaking,
of course, of the magnetism of the physicists and not of the magnetism
of the Mesmers and Cagliostros. Assuredly we possess nothing remotely
like it. What need would the mariner have of a compass, were he