I couldn't have everybody bitten by a radioactive spider or zapped with gamma rays, and it occurred to me that if I just said that they were mutants, it would make it easy. Then it occurred to me that instead of them just being heroes that everybody admired, what if I made other people fear and suspect and actually hate them because they were different? I loved that idea; it not only made them different, but it was a good metaphor for what was happening with the Civil Rights Movement in the country at that time