decorated initial 'M' any Victorian botanists were fascinated by the question of the distribution of plants. Most of them assumed that each species had been created — by some unknown means — in one place only and that plants then migrated from their point of origin to the various places they currently occupied. However, this theory was hard to reconcile with the fact that members of the same group were sometimes found thousands of miles apart with no obvious, natural means by which they could have been transported between their locations. Botanists like Robert Brown, Joseph Hooker and many others hoped that studying the worlds floras in more detail might shed some light on these puzzles and perhaps on the mysterious mechanism by which new species were initially created.

Another motive for investigating plant distribution was that British botany was mainly concerned with taxonomy, the naming and classifying of plants. In the eyes of some nineteenth-century men of science, this was not enough to qualify botany as a science; sciences were supposed to be about mathematics and experimentation, accuracy and precision, but most of all, they were supposed to be concerned with laws. The leading British philosophers of science — men like John Herschel and William Whewell — regarded Newtonian mechanics as the pinnacle of scientific achievement. Newton had discovered a few simple laws that explained everything from the fall of a stone to the motions of the planets; by comparison, botany seemed as intellectually demanding as stamp-collecting. Botanists were acutely aware that their beloved science lacked anything that looked like the laws of physics, and hoped that plant distribution studies would offer them the chance to discover such laws.

As Janet Browne has shown, British plant geographers were partly inspired by the work of Robert Brown, who had observed that the decline in the numbers of species as one sailed away from the equator was characterised by other factors, such as a decline in the size of genera (measured by the number of species each contained). His method of calculating these ratios became known as botanical arithmetic and his ideas were enthusiastically taken-up by the German scientific explorer Alexander von Humboldt. Humboldt travelled extensively in South America, where he observed that Brown's ratios applied not only to changes in latitude, but also to changes in altitude: as one ascended the great mountain chains of South America the lush vegetation of the tropics gave way to a sparse, alpine flora that looked just like the plants of the icy southern tip of the continent.

Joseph Hooker and Charles Darwin were both enthusiastic devotees of botanical arithmetic and hoped that it could be used to develop plant distribution into a rigorous study, one that would eventually lead to the discovery of the laws that explained why particular plants grew where they did. These laws would not only be fascinating in themselves, but would have practical applications too because the British Empire was not just built on industry, but also on plants. Hundreds of plant products were vital to the empire's wealth: rubber, cotton, timber, grains, sugar, tea, oilseeds, spices, indigo, fruit and nuts were just some of them. Understanding plant distribution would allow botanists to predict where valuable, new plants could be found, and also to know which plants could successfully be transplanted to new countries where they could be cultivated profitably.

Last modified 8 March 2008