control of the prickly-pears, Opuntia inermis and 0. stricta, in Australia affords one of the most outstanding examples of the application of biological knowledge to economic purpose. It needs to be recollected that in 1925, about sixty million acres of grazing and farming land were known to be under infestation by prickly-pear in Queensland and New South Wales: the rate of spread of this scourge was stated to be reliably figured at almost one million acres a year. About fifty per cent of the infested territory was under dense prickly-pear, 3-5 ft. high, while the remaining area was affected by scattered infestations of varying intensity. To-day, the enormous rate of increase has ‘been arrested, and less than ten per cent of the former great body of infestation survives: the whole of the primary pear in Queensland and much in New South Wales has broken down and collapsed. Approximately, twenty-five million acres of good land are now cleared and are being developed and brought under production.