The middle class became an important and expanding group in the 19th century. With industrialisation came material gain to the capitalist entrepreneur. New professions came into existence to serve their needs: insurers, engineers, designers. The growth in the population required more architects, lawyers, teachers, doctors, dentists and shopkeepers. Hierarchical tiers emerged within the middle class, each watching each other's status. According to "A New system of Practical Domestic Economy" (1820–1840), being middle class required an income of £150 p.a. or more. In 1851, 3 million out of a total population of 18 million in the UK would have been considered to be middle class.