Social scientists today almost universally believe that essentially, the races of men are largely equal in regard to intelligence. Although there is some disagreement, nonetheless, even among those who disagree, it is agreed that there is not more than approximately one standard deviation difference in the mean of intelligence between any two races. This fact though, is contrary to the basic assumptions of evolution. In order for selection to take place, differences must exist between individual organisms. Selection would then cause races to develop, a process which is called in Darwinian terms, speciation. The lack of important differences between races, especially in the crucial difference between men and "lower" forms of life, viz. intelligence, is seen as a serious difficulty for the evolutionary theory. In addition, the theory of evolution can also be seen as having been an important factor in the extreme forms of racism that existed at the turn of the century and for several years beyond, especially racism against blacks and Jews.