The burrows of worms run down perpendicularly or a little obliquely, and where the soil is at all argillaceous, there is no difficulty in believing that the walls would slowly flow or slide inwards during very wet weather.When, however, the soil is sandy or mingled with many small stones, it can hardly be viscous enough to flow inwards during even the wettest weather; but another agency may here come into play.After much rain the ground swells, and as it cannot expand laterally, the surface rises; during dry weather it sinks again.For instance, a large flat stone laid on the surface of a field sank 3.33 mm.whilst the weather was dry between May 9th and June 13th, and rose 1.91 mm, between September 7th and 19th of the same year, much rain having fallen during the latter part of this time.During frosts and thaws the movements were twice as great.These observations were made by my son Horace, who will hereafter publish an account of the movements of this stone during successive wet and dry seasons, and of the effects of its being undermined by worms.Now when the ground swells, if it be penetrated by cylindrical holes, such as worm- burrows, their walls will tend to yield and be pressed inwards; and the yielding will be greater in the deeper parts (supposing the whole to be equally moistened) from the greater weight of the superincumbent soil which has to be raised, than in the parts near the surface.When the ground dries, the walls will shrink a little and the burrows will be a little enlarged.Their enlargement, however, through the lateral contraction of the ground,willnotbefavoured,butratheropposed,bytheweightofthe superincumbent soil.
Distribution of Worms.--Earth-worms are found in all parts of the world, and some of the genera have an enormous range. They inhabit the most isolated islands; they abound in Iceland, and are known to exist in the West Indies, St.Helena, Madagascar, New Caledonia and Tahiti.In the Antarctic regions, worms from Kerguelen Land have been described by Ray Lankester; and I found them in the Falkland Islands.How they reach such isolated islands is at present quite unknown.They are easily killed by salt-water, and it does not appear probable that young worms or their egg- capsules could be carried in earth adhering to the feet or beaks of land-birds.Moreover Kerguelen Land is not now inhabited by any land- bird.
In this volume we are chiefly concerned with the earth cast up by worms, and I have gleaned a few facts on this subject with respect to distant lands.Worms throw up plenty of castings in the United States.In Venezuela, castings, probably ejected by species of Urochaeta, are common in the gardens and fields, but not in the forests, as I hear from Dr.Ernst of Caracas.He collected 156 castings from the court-yard of his house, having an area of 200 square yards.They varied in bulk from half a cubic centimeter to five cubic centimeters, and were on an average three cubic centimeters.They were, therefore, of small size in comparison with those often found in England; for six large castings from a field near my house averaged 16 cubic centimeters.Several species of earth-worms are common in St.Catharina in South Brazil, and Fritz Muller informs me "that in most parts of the forests and pasture-lands, the whole soil, to a depth of a quarter of a metre, looks as if it had passed repeatedly through the intestines of earth-worms, even where hardly any castings are to be seen on the surface." A gigantic but very rare species is found there, the burrows of which are sometimes even two centimeters or nearly 0.8 of an inch in diameter, and which apparently penetrate the ground to a great depth.
In the dry climate of New South Wales, I hardly expected that worms would be common; but Dr.G.Krefft of Sydney, to whom I applied, aftermaking inquiries from gardeners and others, and from his own observations, informs me that their castings abound.He sent me some collected after heavy rain, and they consisted of little pellets, about 0.15 inch in diameter; and the blackened sandy earth of which they were formed still cohered with considerable tenacity.