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Primary sourceDiary entryc. July 1837

Notebook B 'I think' Tree of Life Sketch

Charles Darwin · c. July 1837

Darwin's private 1837 notebook page with his first branching sketch of evolutionary relationships, headed 'I think'.

Darwin's own handwritten notebook sketch showing the earliest private form of his theory of evolution.

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Handwritten pen sketch by Charles Darwin labeled 'I think' with a branching tree diagram marked with letters A, B, C, D and circled 1, beside a page of cursive handwriting in a curved column, dated 1837 in the top corner.
Charles Darwin, Notebook B, 'I think' tree sketch, c. 1837. Cambridge University Library.

About this source

What it is

This is a page from one of Charles Darwin's private notebooks, written around July 1837, now held at Cambridge University Library. It contains his first sketch of a branching 'tree' showing how species might be related, with the words 'I think' written above it.

Why it matters

This notebook shows Darwin working out his ideas on paper two decades before he published them. The sketch is one of the earliest visual expressions of the idea that species share common ancestors and diverge over time through extinction and gradual change, a private first step toward the theory in On the Origin of Species.

Look closer

  • Why do you think Darwin wrote 'I think' above a scientific diagram?
  • What might the letters and branching lines represent, based on the handwriting beside them?
  • Why would a private notebook be useful evidence for historians studying how an idea developed?

This diary entry is part of the curated material that grounds the AI version of Charles Darwin on Humy.ai — when students ask about related events, the AI can quote it and cite it.

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