Hitler borrowed racism from thinkers like Charles Darwin and Herbert Spencer. Darwin was a natural scientist, who tried to explain the creation of plants and animals through the concept of evolution and natural selection. Herbert Spencer later added the idea of survival of the fittest. According to this idea, only those species survived on earth that could adapt themselves to changing climatic conditions. Darwin never advocated human intervention in what he thought was a purely natural process of selection. However, his ideas were used by racist thinkers and politicians to justify imperial rule over conquered people. The Nazi argument was simple: the strongest race would survive and the weak ones would perish. The Aryan race was the finest. It had to retain its purity, become stronger and dominate the world.