Historians of science have noted that, while Darwin considered the ideas in Wallace's paper to be essentially the same as his own, there were differences. Darwin emphasised competition between individuals of the same species to survive and reproduce, whereas Wallace emphasised environmental pressures on varieties and species forcing them to become adapted to their local conditions, leading populations in different locations to diverge. Some historians, notably Peter J. Bowler, have suggested the possibility that in the paper he mailed to Darwin, Wallace did not discuss selection of individual variations but group selection. However, Malcolm Kottler showed that Wallace was indeed discussing individual variations.