To Francis Galton   8 March [1881]¹

My dear Galton

Very many thanks for your note. I have been observing the immersing tracks, on my walks for several months, & they occur (or can be seen) only after heavy rain. As I know that worms which are going to die (generally from the parasitic larva of a Fly) always come out of their burrows, I have looked out during these months, & have usually found in the morning only from 1 to 3 or 4 along the whole length of my walks. On the other hand I remember having on former years seen scores or hundreds of dead worms after heavy rain. I cannot possibly believe that worms are drowned in the course of even 3 or 4 days immersion; & I am inclined to conclude that the death of sickly (probably with parasites) worms is thus hastened, I will add a few words to what I have said about their tracks, after stating that I f^d only a very few dead ones.² Occasionally worms suffer from epidemics (of what nature I know not) & die by the million on the surface of the ground.—

Your ruby paper answers capitally, but I suspect that it is only by dimming the light, & I know not how to illuminate worms by the same intensity of light & yet of a colour which permits the actinic rays to pass.—³ I have tried drawing triangle of damp paper through a small cylindrical hole, as you suggested, & I can discover no source of error. Nevertheless I am becoming more doubtful about the intelligence of worms.. The worse job is that they will do their work in a slovenly manner when kept in pots, & I am beyond measure perplexed to judge how far such observations are trustworthy.

Ever my dear Galton | Yours most truly | Ch. Darwin