Darwin was a brilliant rational man. When he fell in love with his first cousin Emma Wedgewood he confronted the marriage decision as rationally as he could. Fortunately the Cambridge University Library has The Complete Work of Charles Darwin Online available so we may see Darwin's thought process on this matter.
Darwin wrote in the "Marry" Column:
Children — (if it Please God) — Constant companion, (& friend in old age) who will feel interested in one, — object to be beloved & played with. — —better than a dog anyhow. — Home, & someone to take care of house — Charms of music & female chit-chat. — These things good for one's health. —
Forced to visit & receive relationsbut terrible loss of time. —
WMy God, it is intolerable to think of spending ones whole life, like a neuter bee, working, working, & nothing after all. — No, no won't do. — Imagine living all one's day solitarily in smoky dirty London House. — Only picture to yourself a nice soft wife on a sofa with good fire, & books & music perhaps — Compare this vision with the dingy reality of Grt. Marlbro' St.
In the "Not Marry" column:
No children, (no second life), no one to care for one in old age.— What is the use of working 'in' without sympathy from near & dear friends—who are near & dear friends to the old, except relatives
Freedom to go where one liked — choice of Society & little of it. — Conversation of clever men at clubs — Not forced to visit relatives, & to bend in every trifle. — to have the expense & anxiety of children — perhaps quarelling — Loss of time. — cannot read in the Evenings — fatness & idleness — Anxiety & responsibility — less money for books &c — if many children forced to gain one's bread. — (But then it is very bad for ones health to work too much)
Perhaps my wife wont like London; then the sentence is banishment & degradation into indolent, idle fool —
For Darwin the conclusion was unavoidable.
Marry - Marry - Marry Q.E.D.
They went on to live a long life together, having ten children along the way, although three died at young ages. Emma would be Darwin's constant companion and source of support in it work as well as his caregiver when he was ill, which was quite frequently. One would have to say that Darwin's best theory was the one he followed through with when chose to marry Emma, without whom, he may never have suceeded as he had.
-Professor Walter