Page:The Formation of Vegetable Mould through the Action of Worms (1881).djvu/134

 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, will not be favoured, but rather opposed, by the weight of the 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