A third of a century after Malthus, Darwin attributed biological evolution to survival of only the fittest species (and individuals within species). Though he denied there was any economic significance in his theory of evolution, the economists insisted that superior physical fitness obviously governed economic survival as well. Karl Marx accepted the scientific viewpoints of both Malthus and Darwin when he declared in effect that the working class is the fittest to .survive: they know how to use the tools and to cultivate the fields-the wealthy are parasites. This inaugurated the supranational concept of two world-wide political classes and two competing theories of political organization.