Darwin's theory of evolution by natural selection turned nineteenth-century Christian worldviews upside down. Contradicting the Bible's book of Genesis, the theory claims that living things developed without plan or guidance over untold millions of years. Belief in evolution can coexist with belief in God, yet atheists often credit Darwin with making their breaks with religion possible. Darwin himself slowly abandoned his Christian beliefs, both because of the death of his daughter and the cruelty and wastefulness he observed in nature. By the end of his life, he was a firm agnostic, though he always used great discretion in speaking of his own beliefs.
In his own words: "I cannot persuade myself that a beneficent and omnipotent God would have created the ichneumeonidae with the express intention of their feeding within the living bodies of caterpillars, or that God would have designedly created that a cat should play with mice."