In Darwin's recently published work on vegetable mould, allusion is made to the fact that “In countries where the summer is long and dry, the mould in protected places must be largely increased by dust blown from other and more exposed places.” A few lines further on, however, he states that “In humid countries like Great Britain, as long as the land remains in its natural state clothed with vegetation, the mould in any one place can hardly be much increased by dust.” To this statement no exception can be taken, if we remember that it only applies to a period when the climate is humid, and to a country clothed with vegetation.