Understanding the biogeography of Australia requires knowledge of both existing and former environments. The existing environment is discussed elsewhere in the volume but a few salient points should be reiterated. Three-quarters of Australia is either arid or semi-arid and even much of the better-watered fringes has long dry seasons. Vast stretches of ocean separate it from other land masses except in the north where New Guinea and Indonesia form 'stepping stones' to and from Asia. It is also a low continent with over 99% of its area below 1000 m; even the highest summits barely exceed 2000 m. Since most of the surface has undergone prolonged weath­ ering, poor soils are the rule. The impact of man on the landscape has been less than in more densely settled continents. Aboriginal man has inhabited the continent for 40,000 years or more but agriculture and stock rearing have operated for less than two centuries and the present rural population is sparse. Large parts of the dry interior are not occupied although they are affected to some extent by introduced feral animals.