By his return, he was critical of the Bible as history, and wondered why all religions should not be equally valid. In the next few years, while intensively speculating on geology and the transmutation of species, he gave much thought to religion and openly discussed this with his wife Emma, whose beliefs also came from intensive study and questioning. The theodicy of Paley and Thomas Malthus vindicated evils such as starvation as a result of a benevolent creator's laws, which had an overall good effect. To Darwin, natural selection produced the good of adaptation but removed the need for design, and he could not see the work of an omnipotent deity in all the pain and suffering, such as the ichneumon wasp paralysing caterpillars as live food for its eggs. Though he thought of religion as a tribal survival strategy, Darwin was reluctant to give up the idea of God as an ultimate lawgiver. He was increasingly troubled by the problem of evil.