122 HABITS OF WORMS. Chap. II. 



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 sur- 

 face. 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 op- 

 posed, 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 



* Perrier, ' Archives de Zoolog. exper.' torn. 3, p. 378, 1874. 



