Everywhere man is altering the balance of nature . He is facilitating the spread of plants and animals into new regions, sometimes deliberately, sometimes unconsciously . He is covering huge areas with new kinds of plants, or with houses, factories, slag-heaps and other products of his civilization . He exterminates some species on a large scale, but favors the multiplication of others . In brief, he has done more in five thousand years to alter the biological aspect of the planet than has nature in five million . Many of these changes which he has brought about have had unforeseen consequences . Who would have thought that the throwing away of a piece of Canadian waterweed would have caused half the waterways of Britain to be blocked for a decade, or that the provision of pot cacti for lonely settlers’ wives would have led to Eastern Australian being overrun with forests of Prickly Pear? Who would have prophesied that the cutting down of forests on the Adriatic coasts, or in parts of Central Africa, could have reduced the land to a semi desert, with the very soil washed away from the bare rock? Who would have thought that improved communications would have changed history by the spreading of disease-sleeping sickness into East Africa, measles into Oceania, very possibly malaria into ancient Greece? These are spectacular examples; but examples on a smaller scale are everywhere to be found . We make a nature sanctuary for rare birds, prescribing absolute security for all species; and we may find that some common and hardy kind of bird multiplies beyond measure and ousts the rare kinds in which we were particularly interested . We see, owing to some little change brought about by civilization, the starling spread over the English country-side in hordes . We improve the yielding capacities of our cattle; and find that now they exhaust the pastures which sufficed for less exigent stock .