Robert Brown’s work exemplifies the biogeographer’s efforts to use basic, arithmetical operations and quantified data to compare and make arguments about variation in organic phenomena and the relationship of this variation to geographic and climactic conditions. In the Botany of Terra Australis, for example, Brown tests the assertion commonly held by nineteenth century botanists that dicotyledonous plants outnumber monocotyledonous plants by examining whether climate affects the numbers of either type in the general botanical population: 5
With a view to determine how far the relative propositions of these two classes [dicotyledons and monocotyledons] are influenced by climate, I have examined all the local catalogues or Floras which appear most to be depended on. . . . The general results of this examination are that from the equator to about 30° of latitude, in the northern hemisphere at least, the species of dicotyledonous plants are to monocotyledonous plants as about 5 to 1 . . . and that in the higher latitudes a gradual diminution of dicotyledonous takes place, until at about 60° N. lat. and 55° S. lat. they scarcely equal half their intratropical proportion. (Miscellaneous Botanical Works 8)
In the passage Brown draws on quantitative descriptions of location and previously tabulated statistics on the number of species of each sort of plant as well as calculated ratios to describe the limits of the geographical distribution of dicotyledonous and monocotyledonous plants. He concludes, based on the data and calculations, that dicotyledonous plants are abundant near the equator but become less abundant in northern latitudes. This conclusion provides precise quantitative detail supporting what was otherwise an anecdotal assumption about the difference in dicotyledonous and monocotyledonous plants in the general population of flora. It also supplies new information about the relationship between location and the thriving of dicotyledonous plants, which was previously unknown.
Darwin and the Biogeographers
The quantitative data and mathematical methods used by Brown and other biogeographers inspired Darwin to develop quantitative/mathematical arguments about the phenomena of variation and evolution. Evidence that Darwin was inspired by their methods can be found in his reading habits, in the private thoughts he recorded in his notebooks, and in his letters discussing the construction of his arguments for The Origin of the Species.
A brief assessment of Darwin’s reading habits during the development of The Origin of the Species reveals that Darwin was familiar with the central works of biogeography and read them as he was developing his ideas for his masterwork. He was most likely introduced to them by his mentor John Henslow, who presented him, before he left Cambridge, with Humboldt’s Personal Narrative of Travels to the Equinoctial Regions of the New Continent during the Years 1799–1804 (1814–1829) in which the explorer discusses the importance of quantification in the discovery of natural laws and offers statistics on temperature, altitude, longitude and latitude, etc. for various regions and flora in South and Central America (Schweber 205). It is clear that Darwin read this book and found it important because his copy contains extensive notes. According to the compiler of Darwin’s marginalia, Mario A. Di Gregorio. Humboldt got Darwin “thinking about the distribution and the relation of organism to organism in the context of isolation, extinction, and the breeding of wild and domesticated animals” (xxxv). In addition to Humboldt, Darwin also owned multiple editions of Lyell’s Principles of Geology (1st, 5th, 6th, 7th, 9th, 10th, and 11th) (Di Gregorio 530–44). In the fifth edition, volume three, chapter five, “Laws which Regulate the Distribution of the Species,” discusses at length the important figures of biogeography and their work. Like Humboldt’s Personal Narrative, Darwin marked and commented heavily on the text, providing evidence for his interest in their ideas (Di Gregorio 535–36).
When Darwin returned home and began developing the ideas that would be presented in The Origin of the Species, he continued to show interest in the work of biogeographers, as references to their works and ideas in his notebooks testify. At the end of Notebook C, in a section titled, “To be Read,” for example, Darwin lists “Brown at end of Flinders & at end of Congo voyage, Decandolle. Philosophie, or Geographical Distribution. <<in Dict. Sciences. Nat. in Geolog Soc.>>” as texts which he believed would be beneficial to the development of his ideas (268).6 Of these two works, Alphonse de Candolle’s “Essai Elementaire de Geographie Botanique” was of particular importance to developing his ideas in The Origin of the Species. Of all the books listed in Gregorio’s Charles Darwin’s Marginalia, Darwin’s copy of “Gèographie Botanique” has the most marginalia, prompting Di Gregorio to write that the work “seems to be a catalyst for much thinking around distribution, the struggle for existence, isolation, and consequently selection” (xxxiv).
Not surprisingly, the writings in the notebooks themselves echo the biogeographers’ sentiments about the importance of developing a quantitative/mathematical approach to the study of plant and animal life. In Notebook D, for example, Darwin echoes Humbolt’s sentiment that investigators of organic phenomena could and should emulate the quantitative inductive methods used in celestial and terrestrial physics:
Astronomers might formerly have said that God ordered each planet to move in its particular destiny.—In the same manner God orders each animal created with certain form in certain country, but how much more simple, & sublime power let attraction act according to certain laws such are inevitable consequent let animal be created, then by fixed laws of generation, such will be their successors.—let the powers of transportal be such and so will be the form of one country to another.—let geological changes go at such a rate, so will be the numbers & distribution of the species!! (Notebook B, 101–102)
In this passage, Darwin reveals a clear link in his thinking between the methods of astronomy and the study of organic phenomena ascribed to by biogeographers. Just as astronomers create general laws by quantifying the period of the revolution of celestial bodies, their distance from one another, the amount of space they sweep out in a given period, etc., the biogeographer could arrive at quantitative laws describing the number and distribution of the species by determining the rate of generation of organisms, the rate in change of geological conditions, and the power of transport.
Darwin’s reading habits and his notebooks supply evidence that he was considering the idea that natural laws might be discovered by working inductively from quantified evidence as he was gathering his thoughts for what would become The Origin of Species. However, it was not until the mid-1850s, when he began writing the manuscript of Natural Selection (his “big species book”), that there is evidence that Darwin began to put these ideas into practice by gathering statistical data and making arithmetical calculations to test his theory of evolution and build his arguments.
In 1855 Darwin began lengthy correspondences with Asa Gray and H. C. Watson in which he asked the botanists to supply him with information about genera and species, and discussed with them calculations of the ratios of varieties to genera in large and small genera. The quantified data and mathematical calculations discussed in these letters serve as sources of invention and argument for a relationship of descent and the principle of divergence of character in The Origin of the Species.
In his second letter to Asa Gray, on August 24, 1855, for example, Darwin hints to Gray that if he could get a reliable systematist to help him identify “close species,” (i.e., species that closely resemble one another) he could calculate whether there was a propensity for larger genera to have more of these types of species than smaller genera:
It occurred to me that if I could get some good systematists . . . to mark (without the object being known) the close species in a list; then if I counted the average number of the species in such genera, & compared it with the general average . . . of the species to the genera in the same country; it would, to a certain extent, tell whether on average the close species occurred in the larger genera. (Darwin to Gray, August 24, 1855)
Darwin makes a similar request to Watson, who obliges him by marking close species in his catalogues of English plants (Watson to Darwin August 17, 1855).