Natural selection does appear, as Darwin thought, to be the main driving force of evolution. Karl Marx and Friedrich Engels pointed out that Darwin was severely biased by his views as a well-to-do member of the English bourgeoisie. If a number of animals or plants in a population carry a gene which makes them fitter than the rest of it, in the sense that on the average they leave more descendants behind them, that gene will tend to spread through the population. Economic forces determine the development of human societies, as Marx first clearly saw. But whereas the earlier economists, such as Adam Smith, thought that economic competition would necessarily make all nations richer, we now see that this is not true. Hitler is dead, but his ideas are alive, and we must be very careful to see that Darwinism is not made the basis of a new Hitlerism.