Earthy stuff . . .

The first winds off the sea were only middling fierce, but strong enough to pluck the leaves from the hillside's hawthorns. Even after all that whipping and shuddering, the bare branches were left with such a rich crust of berries that the hedges are brilliantly crimson, here with haws and there with fuchsia blossom.

Hawthorn leaves, rich in nitrogen and sugars, are among the favourites of the big common earthworm, Lumbricus terrestris, so it's an ill wind, etcetera. Ash and apple are other favourites, but these seem to have hung on, together with the birch, another one with sugary foliage. Oak and beech, too, would rather snap a branch than give up their leaves in September, and that suits the worms, which find them rather too rich in tannins for their taste, at least until they have weathered.

As for conifer needles, these are loaded with distasteful polyphenols, which is why they are left to pile up into such a thick carpet and take so long to break down into humus. But while earthworms may know what they like, as demonstrated in elegant scientific experiments, their day-to-day behaviour doesn't always seem to match up.

The other day a Wicklow reader, sweeping the pathways of his Enniskerry garden, noted pine needles sticking up from worm-holes. They were the two-pronged needles from Scots pine, and each hole had several pairs, pulled down by the base, with their tips left sticking up about an inch above the ground.

He was intrigued enough to experiment. He swept the path clear of needles and then, at evening, scattered a few dead holly leaves around. Next morning, each worm-hole had one of the prickly leaves protruding in the same way. What exactly, he has asked me, was going on?

Charles Darwin would have warmed to him. This sort of behaviour by earthworms, and its vital consequences for much of the planet's fertility, was one of his lifelong interests: it is no less significant today.

In 1837, Darwin was only a year or so home from the voyage of the Beagle when he visited his Uncle Jos in rural Shropshire. Jos showed him a plot of disused land where a marl of lime and cinders, spread years before, had disappeared into the soil, leaving a layer of loam above it; he thought perhaps worms had done it.

From this casual observation, followed up by much digging of holes in fields and detailed measurement, Darwin wrote his first five-page article on the role of earthworms in forming the topsoil of England. "The whole operation," he declared, " is due to the digestive process of the common earthworm."

Most of the next 40 years were spent in the anguish and tumult of presenting his ideas on the evolution of species. But in his final decade, exhausted by religious controversy, he retired to less contentious projects postponed from his youth. At his country house in Kent, the billiard room filled up with glass-covered pots of earth as he explored the senses of earthworms, flashing lights at them, waving a red-hot poker over them, blowing tobacco smoke at them, testing their preferences in vegetables, or recruiting his family to serenade them with whistle, piano and bassoon.

"Worms are poorly provided with sense-organs," he concluded in his last book, Vegetable Mould and Earthworms, published in 1881, a year before he died. Their sense of touch alone was well developed, and they seemed to "enjoy the pleasure of eating". But what really surprised him was that they seemed to show "some degree of intelligence, instead of a mere blind, instinctive impulse, in their manner of plugging up the mouths of their burrows.

"They act in nearly the same manner as would a man who had to close a cylindrical tube with different kinds of leaves, petioles, triangles of paper, &c., for they commonly seize such objects by their pointed ends . . . Worms drag an infinite number of dead leaves and other parts of plants into their burrows, partly for the sake of plugging them up and partly as food."

He described their great muscular power and how, in places, "more than ten tons (10,516 kilos) of dry earth annually passes through their bodies and is brought to the surface on each acre of land." The leaves dragged into the burrows are shredded, partially digested and mixed with earth "like a gardener who prepares fine soil for his finest plants". The burrows, often several feet deep, let air into the soil, help its drainage and make a path for plant roots.

There are now thousands of scientific papers parsing every aspect of the beneficial life of earthworms. If today they are seen dragging all sorts of materials underground, including shreds of plastic and synthetic cloth, this simply reflects what we have done to their original woodland habitat, in which every kind of litter, having lived itself, would give up its substance to the soil.

With so much well-drained permanent pasture, Ireland must still be one of the most hospitable habitats in Europe, with many areas perhaps not far short of the 53,767 worms per acre that Darwin proposed for the pastures of Kent. Our flourishing populations of rooks, foxes and badgers (where they are let alone) all reflect this high density.

How far this will change with the insidious spread of the New Zealand flatworm, Artioposthia triangulata, is impossible to predict. This is the appalling creature, originally imported in egg form on the rootstock of plants from New Zealand and now widespread in Irish garden centres and nurseries and in the countryside beyond. It crawls into burrows after worms and smothers them with corrosive digestive juices in order to eat them. There has to be some biological control of Artioposthia for Ireland to use, but no one has found it yet. It is a modern menace, made inevitable by the global traffic in plants between one biosphere and another. Charles Darwin brought a whole box of Galapagos plants back in the Beagle (though as specimens, not growing in pots) and was left red-faced when botanists found them beautiful and wanted to know more about their habitat: "I knew no more about the plants, which I had collected, than the Man in the Moon."