CLIMATE AND HUMAN HISTORY but there is little doubt that by proper attention to the ecology and genetics of grasses we could double the output of the world's pastures. For one thing, proper dosing with mineral salts helps the growth of plants which make greater demands on the soil, and so takes the ecological succession a stage further to a richer herbage. In dry areas it often helps also by conserving more moisture in the soil. Then there are strange and subtle interrelations between grass and the beasts that eat it. Their trampling and their browsing alter condi- tions for the herbage. Too little grazing may allow scrub or moor to invade the pasture; too much may impoverish the sward. Such problems are especially prominent in new countriesin New Zealand, for instance, there seem to have been no indigenous grazing creatures, save possibly the giant flightless bird, the moa; yet to-day 94 per cent, of the country's exports are the products of grass-eating animals. Here, to clear scrubland for sheep, not only must the scrub be cut and rooted up and burned, but cattle must be introduced to keep the bracken and brush from winning back the land they have lost. As Dr. Stapledon says, " Cattle, no matter how prices rule, are essential to the reclamation and maintenance of scrublands. They are imple- ments as necessary to the wool grower on hilly, scrubby country as the plough to the producer of wheat on the plains." Trampling,, too, prevents the grass from getting coarse and rough. The amount of grazing a pasture will stand depends a good deal on climate. If grass- land (as in so much of Europe and New Zealand) is not the natural climax of plant life, but is only a "sub-climax," which would go on to a richer type of vegetation, such as forest, if left to itself, then it will stand very heavy grazing. If, however, the climate is so dry that grass of sorts is the natural climax, it has fewer reserves, so to speak, and heavy grazing may seriously damage it. But the amount of grazing will also depend on the kinds of grasses there are to be grazed. In New Zealand the native vegetation, un- used to being nibbled down to the ground, succumbs to this new treatment. A judicious mixture of the right grasses and clovers from all over the world (only we must remember that what is right for one place may be very wrong for another!) is rapidly raising the pro- ductive power of grass. This will soon get to a limit; but then the geneticist can step in and continue the process by deliberately breed- ing richer and more resistant pasture plants. A beginning has been made with this at places like the Grass Research Station at Aberyst- wyth, and the results already obtained, together with the comfortable knowledge of what has been actually achieved with wheat, warrant great hopes for the future.