Normal distributions, expressible in the form formula_1, arose in various works on probability and the theory of errors. Belgian sociologist and statistician Adolphe Quetelet discovered that its extremely wide applicability in his analysis of vast amounts of statistics of human physical characteristics such as height and other traits such as criminality and alcoholism. Queletet derived the concept of the "average man" from his studies. Sir Francis Galton employed Quetelet's ideas in his research on mathematical biology. In his experiments with sweet peas in the 1870s, Galton discovered that the spread of the distributions of a particular trait did not change over the generations. He invented what he called the "quincunx" to demonstrate why mixtures of normal distributions were normal. Galton noticed that the means of a particular trait in the offspring generation differed from those of the parent generation, a phenomenon now known as regression to the mean. He found that the slopes of the regression lines of two given variables were the same if the two data sets were scaled by units of probable error and introduced the notion of the correlation coefficient, but noted that correlation does not imply causation.