A vast girdle of rain-forest encircles the earth between the tropics. It is bisected somewhat unequally by the equator, so that rather more of its area lies in the northern than in the southern hemisphere. Not only is the forest interrupted in many places by mountain ranges and plateaux but, in the more densely populated regions of the tropics, it has been replaced so completely by cultivation that it is impossible to reconstruct the original climax vegetation (Richards, 1952).