und are not common. For two winters I visited
all the decrepit almond-trees at hand, inspected them all at the base of
the trunk, under the jungle of stubborn grasses and undergrowth that
surrounded them; and how often I returned with empty hands! Thus my
hundred and fifty butterflies had come from some little distance;
perhaps from a radius of a mile and a quarter or more. How did they
learn of what was happening in my study?

Three agents of information affect the senses at a distance: sight,
sound, and smell. Can we speak of vision in this connection? Sight could
very well guide the arrivals once they had entered the open window; but
how could it help them out of doors, among unfamiliar surroundings? Even
the fabulous eye of the lynx, which could see through walls, would not
be sufficient; we should have to imagine a keenness of vision capable of
annihilating leagues of space. It is needless to discuss the matter
further; sight cannot be the guiding sense.

Sound is equally out of the question. The big-bodied creature capable of
calling her mates from such a distance is absolutely mute, even to the
most sensitive ear. Does she perhaps emit vibrations of such delicacy or
rapidity that only the most sensitive microphone could appreciate them?
The idea is barely possible; but let us remember that the visitors must
have been warned at distances of some thousands of yards. Under these
conditions it is useless to think of acoustics.

Smell remains. Scent, better than any other impression in the domain of
our senses, would explain the invasion of butterflies, and their
difficulty at the very last in immediately finding the object of their
search. Are there effluvia analogous to what we call odour: effluvia of
extreme subtlety, absolutely imperceptible to us, yet capable of
stimulating a sense-organ far more sensitive than our own? A simple
experiment suggested itself. I would mask these effluvia, stifle them
under a powerful, tenacious odour, which would take complete possession
of the sense-organ and neutralise the less powerful impression.

I began by sprinkling naphthaline in the room intended for the reception
of the males that evening. Beside the female, inside the wire-gauze
cover, I placed a large capsule full of the same substance. When the
hour of the nocturnal visit arrived I had only to stand at the door of
the room to smell a smell as of a gas-works. Well, my artifice failed.
The butterflies arrived as usual,