In vol. 1 of this work written after Spencer read Darwin's On the origin of species, Spencer originated the express "survival of the fittest." Spencer conceived that every species is endowed with its own type of physiological unit, each unit being capable, under certain circumstances, of reproducing the whole organism. Spencer set forth doctrines of evolution some years before the appearance of the Origin of species. 

"Spencer's second book, Principles of Psychology, published in 1855, explored a physiological basis for psychology, and was the fruit of his friendship with Evans and Lewes. The book was founded on the fundamental assumption that the human mind was subject to natural laws and that these could be discovered within the framework of general biology. This permitted the adoption of a developmental perspective not merely in terms of the individual (as in traditional psychology), but also of the species and the race. Through this paradigm, Spencer aimed to reconcile the associationist psychology of Mill's Logic, the notion that human mind was constructed from atomic sensations held together by the laws of the association of ideas, with the apparently more 'scientific' theory of phrenology, which located specific mental functions in specific parts of the brain.^[12]

Spencer argued that both these theories were partial accounts of the truth: repeated associations of ideas were embodied in the formation of specific strands of brain tissue, and these could be passed from one generation to the next by means of the Lamarckian mechanism of use-inheritance. The Psychology, he believed, would do for the human mind what Isaac Newton had done for matter.^[13] However, the book was not initially successful and the last of the 251 copies of its first edition were not sold until June 1861." (Wikipedia article on Herbert Spencer, accessed 8-2021).

Spencer founded and edited this series.