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In 1838, before proposing to his cousin Emma Wedgwood, Charles Darwin drew up two columns headed 'Marry' and 'Not Marry.' He weighed a 'constant companion' and 'charms of music and female chit-chat' against loss of time for books and fewer visits to clever men at clubs, then concluded 'Marry—Marry—Marry Q.E.D.' The episode shows him applying his naturalist's habit of systematic weighing to a decision he then made wholeheartedly.
Soon after their 1839 wedding, Emma wrote Charles a letter saying she feared his religious doubts might separate them for eternity. She asked him not to reject faith without honest consideration, and said everything that concerned him concerned her. Charles kept the letter all his life and wrote on it: 'When I am dead, know that many times, I have kissed and cryed over this.' The episode shows a disagreement about ultimate things voiced openly between spouses and received as an act of love rather than an attack.
The book argues the Darwins' marriage held because each named the gulf honestly and neither made the other's change of mind a condition of love. Emma kept her faith and her fear for his soul, and Charles kept his doubt. What bound them was daily devotion, shared nursing of sick children, and shared grief, especially over their daughter Annie's death in 1851, which deepened his loss of belief. Deep disagreement is survivable when it is acknowledged rather than suppressed, and when the relationship is carried by shared practice rather than shared doctrine.
Emma read and criticized drafts of Origin to help Charles make his arguments clearer, and after his death she cut private passages on religion from his autobiography. Seen this way, a believing spouse served first as a critical reader who sharpened his case, and only later, in a separate role, as the one who decided what of his private religious views the public would see.