technology. Its engineering firms were in worldwide demand for designing and constructing railways. Ease of discovery and rate of progress A necessary part of understanding scientific progress is the ease of scientific discovery. In many cases, from planetary science to mammalian biology, the ease of discovery since the 1700s and 1800s can be fitted to an exponentially decaying curve. But the rate of progress is also dependent on other factors, such as the number of researchers, the level of funding, and advances in technology. Thus the number of new species of mammals discovered between the late 1700s and late 1800s followed grew exponentially before leveling off in the 1900s; the general shape is known as the logistic curve. In other cases, a branch of study reached the point of saturation. For instance, the last major internal human organ, the paraythyroid gland, was discovered in 1880 by Ivar Viktor Sandström. This does not mean that basic science was coming an end. Despite the despondency of many Victorian-era scientists, who thought that all that remained was measuring quantities to the next decimal place and that new discoveries would not change the contemporary scientific paradigm, as the nineteenth century became the twentieth, science witnessed truly revolutionary discoveries, such as radioactivity, and basic science continued its advance, though a number of twentieth-century scientists shared the same pessimism as their late-Victorian counterparts. Mathematics and statistics In the field of statistics, the nineteenth century saw significant innovations in data visualisation. William Playfair, who created charts of all sorts, justified it thus, "a man who has carefully investigated a printed table, finds, when done, that he has only a very faint and partial idea of what he has read; and that like a figure imprinted on sand, is soon totally erased and defaced." For example, in a chart showing the relationship between population and government revenue of some European nations, he used the areas of circles to represent the geographical sizes of those nations. In the same graph he used the slopes of lines to indicate the tax burden of a given population. While serving as nurse during the Crimean War, Florence Nightingale drew the first pie charts representing the monthly fatality rates of the conflict, distinguishing deaths due to battle wounds (innermost section), those due to infectious disease (outer section), and to other causes (middle section). (See figure.) Her charts clearly showed that most deaths resulted