Down, January 21st, 1881.
My Dear Sir,
I am much obliged for your very interesting letter. Your results appear to me highly important, as they eliminate one means by which animals might perhaps recognise direction; and this, from what has been said about savages, and from our own consciousness, seemed the most probable means. If you think it worth while, you can of course mention my name in relation to this subject.
Should you succeed in eliminating a sense of the magnetic currents of the earth, you would leave the field of investigation quite open. I suppose that even those who still believe that each species was separately created would admit that certain animals possess some sense by which they perceive direction, and which they use instinctively. On mentioning the subject to my son George, who is a mathematician and knows something about magnetism, he suggested making a very thin needle into a magnet; then breaking it into very short pieces, which would still be magnetic, and fastening one of these pieces with some cement on the thorax of the insect to be experimented on.
He believes that such a little magnet, from its close proximity to the nervous system of the insect, would affect it more than would the terrestrial currents.
I have received your essay on Halictus ("Sur les Moeurs et la Parthénogénèse des Halictes" ("Ann. Sc. Nat. " IX., 1879-80).), which I am sure that I shall read with much interest.