For each tendency of society to produce negative selections, Darwin also saw the possibility of society to itself check these problems, but also noted that with his theory "progress is no invariable rule." Towards the end of "Descent of Man", Darwin said that he believed man would "sink into indolence" if severe struggle was not continuous, and thought that "there should be open competition for all men; and the most able should not be prevented by laws or customs from succeeding best and rearing the largest number of offspring", but also noted that he thought that the moral qualities of man were advanced much more by habit, reason, learning, and religion than by natural selection. The question plagued him until the end of his life, and he never concluded fully one way or the other about it.