Gray abhorred slavery. In his view, science proved the unity of all man because all human races can interbreed and produce fertile offspring, i.e., all members of a species are connected genetically. He also felt Christianity taught the unity of man. He was not an abolitionist, feeling it was more important to preserve the Union, yet he also felt if the South persevered in the conflict, every slave should be set free, by use of more force if necessary. However, he was not sure what should be done with the freed slaves. This was another area where Gray and Agassiz disagreed; Agassiz felt each race had different origins.