2 2o THE DARWINIAN THEORY 



on the "Formation of Mould," in which he called 

 attention to the characters of vegetable mould or 

 earth ; to its homogeneous nature, whatever was the 

 character of the subsoil ; and to the uniform fineness 

 of its particles. He gave the history of some fields, 

 some of which a few years before had been covered 

 with lime, and others with burnt marl and cinders, 

 and showed that on cutting into the soil, the cinders 

 or lime were found, in a fairly uniform layer, some 

 inches below the turf — as though, to quote the 

 farmer's opinion, the fragments had worked them- 

 selves down. 



The explanation he offered, originally suggested 

 by his uncle, Mr. Josiah Wedgwood, was that earth- 

 worms, by bringing earth to the surface in their 

 castings, must undermine any objects lying on the 

 surface, and thus cause an apparent sinking. 

 According to his habit, Darwin continued steadily 

 to accumulate observations, to devise experiments, 

 and to collect information from all possible sources. 

 Finally, in 1881, or forty-four years after his first 

 paper on the subject was published, his last book — 

 the " Formation of Vegetable Mould through the 

 Action of Worms " — was published. He describes 

 himself as getting almost foolishly interested in it ; 

 and when the book was published, it was received 

 with what he describes as " almost laughable 

 enthusiasm," 8000 copies being sold in about three 

 months. 



Earthworms can live anywhere in a layer of earth, 

 even if thin, provided it retains moisture ; dry air 

 is fatal to them, but they can live submerged in 



