from disease, which led the general public to demand improved sanitation at field hospitals. Although bar charts representing frequencies were first used by the Frenchman A. M. Guerry in 1833, it was the statistician Karl Pearson who gave them the name histograms. Pearson used them in an 1895 article mathematically analyzing biological evolution. One such histogram showed that buttercups with large numbers of petals were rarer. Normal distributions, expressible in the form , 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. Quetelet 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. During the late nineteenth century, British statisticians introduced a number of methods to relate and draw conclusions from statistical quantities. Francis Edgeworth developed a test for statistical significance that estimated the "fluctuations"—twice the variance in modern language—from two given means. By modern standards, however, he was extremely conservative when it comes to drawing conclusions about the significance of an observation. For Edgeworth, an observation was significant if it was at the level of 0.005, which is much stricter than the requirement of 0.05 to 0.01 commonly used today. Pearson defined the standard deviation and introduced the -statistic (chi-squared). Pearson's student, George Udney Yule, demonstrated that one could compute the regression equation of a given data set using the method of least squares. In 1828, miller and autodidactic mathematician George Green published An Essay on the Application of Mathematical Analysis to the Theories