Here is a Website that contains Darwin's field notebooks and an interesting introduction to them.
A quote by Darwin from the Website: "[A naturalist] ought to acquire the habit of writing very copious notes, not all for publication, but as a guide for himself. He ought to remember Bacon's aphorism, that Reading maketh a full man, conference a ready man, and writing an exact man; and no follower of science has greater need of taking precautions to attain accuracy; for the imagination is apt to run riot when dealing with masses of vast dimensions and with time during almost infinity."
This does not hold just for naturalists—I think.
No further comment!
1. See also Darwin on collecting notes with the intention not to publish and Confronting Disorderly Impressions