Radical approaches to history depended to some extent on the burgeoning professionalization of the discipline because a systematic understanding of source materials helped generate new questions about their use and limitations. But larger developments played their part, too. The growth and new concentrations of population in cities, the scale of immigration into the United States in particular, the pace of industrialization in North America and western Europe after 1850, the shrinkage of the world in space and time99-these implications of a mass society caught in a period of technological turmoil could not escape historians looking for interesting ways to think about the past. The new context implied that economic issues had lain for too long at the margin of enquiry. It asked how the histories of economies and the social groupings that interacted with them should be constructed. It raised with renewed force the Enlightenment’s questions about history’s relation to social science. Nor did historians need to frame all their own questions ab initio. For the second half of the nineteenth century became so heated with argument about social theory that even if historians had never read a word of Marx or Dilthey or Pareto or Durkheim or Weber, even if they had turned their backs on the sour Methodenstreit which so disfigured German historical discussion in the 1890s, they could hardly remain immune from the contagion of self-awareness and self-criticism which infected most areas of intellectual enquiry.100 It did not help those seeking isolation that they comprised the last generation of universalists who expected to find the time to read about the latest developments in biology and geology, physics and philosophy as well as to sustain their historical projects. Epidemics travelled quickly.