In these letters to Caroline Watts, a young friend and teacher in Durham, England, Spencer writes principally about the teaching of geometry and astronomy. He also comments on current political and literary events in England, including John Bright's defeat, the Corn Laws, and theories about health and religion. There is frequent mention of his brother Thomas Spencer, and his son Herbert Spencer.

This is a miscellaneous collection of letters, to and from Lubbock, concerning his lectures and his interest in paleontology, as well as references to political matters, particularly the Irish question, and numerous mentions of the legislative bills with which he was involved. There are letters concerning his publications, "Prehistoric Times" (1865), and "Sense and Intelligence of Animals" (1888).