Henrietta Darwin. Credit: Darwin Correspondence Project

A small, lockable leather diary - kept in the vast archives of Cambridge University Library - has led to a reassessment of one of the key relationships in Charles Darwins life.

The Darwin Correspondence Project (www.darwinproject.ac.uk), with the permission of Darwins family, is making public for the first time the short, intense and hugely revealing personal journal of Darwins daughter Henrietta.

Although many members of Darwins family helped edit his work over the years, it was probably Henrietta upon whom Darwin most relied. Indeed, she was a key editor of The Descent of Man and Selection in Relation to Sex, published in 1871.

In a rare glimpse into the intellectual life of the women in Darwins circle, Henrietta reflects in her diary on a wide-ranging debate with one of her cousins, Frances Julia Wedgwood (known to the family as Snow), on free will, good and evil, and the possibility of eternal life.

Both women demonstrate a striking sophistication of thought in their differing assessment of the implications for religious thinking of Darwins Development Theory, but although Henriettas arguments are reminiscent of Darwins own discussion of religious belief in his correspondence and other writings, her journal reveals a thoughtful woman reasoning her way to a humanistic position that is very much her own.

The diary, published today for the first time as an appendix to The Correspondence of Charles Darwin, Volume 19, was written over the period March to July 1871, the year of 28-year-old Henriettas sudden marriage to a man she had known for less than three months: it introduces a confident, intelligent, reflective and passionate young woman.

Dr. Alison Pearn, Associate Director of the Darwin Correspondence Project, said: To anyone who has only met her as the overly anxious hypochondriac aunt in Gwen Raverats Period Piece, this Henrietta is unrecognizable.

1871 was also the year in which Henriettas father published his most enduringly controversial work The Descent of Man and Selection in Relation to Sex. Taken together with Darwins letters from the period, the diary dramatically overturns previous perceptions of her character and role.

Often dismissed as a conservative or even a negative influence on her fathers work, driven by a desire for respectability to acts of censorship, Henrietta emerges instead as a free-thinker. She was sceptical of the contagious emotion of evangelical missions, and unable to reconcile the idea of a personal, benevolent God either with the concept of free will or with the suffering and wastefulness of the natural world.

One extract reads: How can she (Snow) shake off the bonds of reason  reason tells us plainly that each life is not ordered for its own good If we cannot know goodness how can we recognise in which part of our nature God is revealed This view seems to cast what few convictions I have to the winds.

However, Henrietta did not only ponder the religious ramifications of her beliefs through the journal; the extracts published today also include heartfelt entries concerning her blossoming love life.

In June 1871 Henrietta met Richard Buckley Litchfield, a barrister and lecturer in music at the London Working Mens College. They were married in the parish church in her parents village of Downe, Kent, on 31 August.

Dr. Pearn said: The intimate and deeply reflective journal entries from July, which hint at originally unrequited passion, cover the period of their courtship. Reading at times like teenage romantic fiction, with all the agonies of uncertainty and longing, it also dissects Henriettas reasons for choosing to be married in churcha decision she did not reach lightly  and her sadness at leaving her parents.

Another extract (July 4, 1871) reads: A beautiful day  rainy gleams all over the field making it look like the paradise he called it. I am glad it is fine. I want to welcome him if he comes as I think he comes I think he must care  it cant be only that he thinks I should be a nice sort of person to marry.

Meanwhile, Henriettas entry for July 9, 1871, underlines how she agonised over whether to marry in church or a registry office.

It reads: I want to think why I should like to be married in a church. I should feel a registry office very incomplete I should not be content at the supreme moment of my life without some mark of its solemnity  some outward and visible sign of the inward and spiritual grace.

Can I vow to love honor & obey  The two last, yes. The first I think so. Is it love to think about him day & night  when I wonder what he thinks on every conceivable subject  when I feel my day made happy & bright by one short letter. I want him to take me in his arms & say I shall never leave him. I long for him to strike the match which is to kindle me. The fire is laid but I cant set it alight.

Added Dr. Pearn: Weve been able to look more closely at Henriettas life as part of a larger research project on Darwin and Gender and Im very pleased we have this chance to rehabilitate her. Her contribution to Darwins work, in particular on religion, has been seen as purely negative  editing out rather than editing in any constructive sense. Now we can see her as her father saw her, as a dear coadjutor & fellow-labourer, and as a lively member of an intellectually stimulating household.