In the novel 4.50 from Paddington, Dr Morris is a retired doctor, who had attended the Crackenthorpe family in the past. He had been Dr Quimper's senior partner, and had retired soon after Dr Quimper joined him.
When Luther Crackenthorpe suffered a gastric attack at Christmas time, he told Dr Quimper that he had had similar attacks before, when there was too much rich food. Dr Quimper wrote to Dr Morris to find out more about these attacks, as he felt that certain indications were more consistent with arsenic poisoning than with gastro-enteritis. However, Dr Morris felt that Dr Quimper was being foolish.
Dr Morris later gives Inspector Craddock some information about the Crackenthrope family. He says that Josiah Crackenthorpe was sane enough, but that his wife was neurotic. He was of the opinion that Luther had inherited a certain instability from her. He thought Luther resented that fact that his father was disappointed in him, and brooded over it. He also thought it possible that Luther had never felt very adequate as a man, and resented his financial position, having an income but having no power of appointment of capital, and so he disliked his own sons heartily.