In "The Ascent of Man" (a title he used long before Jacob Bronowski made his famous television series of the same name), he compares the ideas of Charles Darwin and Herbert Spencer in particular, with reference to Benjamin Kidd. The book is notable for his theory that the ills of society, and in particular mental illness, are caused by the increasing demands of society on the human organism which is adapted for life in small, unpoliced, communities of equals, rather than the constraining power structures of modern societies which leave man anything but free. This line of thought would later be expressed by Boris Sigis and Wilfred Trotter, and by Sigmund Freud in "Civilisation and its Discontents"