Since the first half of the nineteenth century, it has been known that certain species of protozoa will grow in a suitable medium inoculated with soil. In spite of this, however, some later workers have asserted that such organisms did not occur in normal soils, while others, though recognising their presence, denied the existence of active forms. To Martin and Lewin (28) must be ascribed the distinction of first showing that amœbæ and flagellates are present in field soil in a trophic condition; and owing to the labours of numerous investigators, it is now known that most soils contain these organisms; but unfortunately there is little or no knowledge as to their behaviour in natural conditions.