Complex sentence: He soon commissioned Gould to write "The Mismeasure of Man", but it was not until three years later, when Gould accumulated 33 columns, that it occured to either of them that the "Natural History" columns should be published in a single volume.
Split into simpler sentences: He soon commissioned Gould to write "The Mismeasure of Man". It wasn't until three years later, when Gould accumulated 33 columns, did it occur to either of them that the columns should be published in a single volume.