•Why it earns a slot
Why it earns a slot: writing three years after Darwin's Origin, Spencer tried to stretch evolution into a single law covering nebulae, embryos, languages, and governments, while using the Unknowable to make peace between religion and science.
Herbert Spencer tries to reconcile religion and science by proving both bow to the same unknowable power, then builds one law, evolution, to explain the entire universe.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.