Darwin's First Botanizing Steps Followed His Geological Ones

 “I collected every plant, which I could see in flower, & as it was the flowering season I hope my collection may be of some interest to you." - Charles Darwin in a letter to his friend and mentor John Stevens Henslow, 1836.

Charles Robert Darwin's interest in the natural world was widespread. As a student, he loved to hunt animals and collected bugs and minerals. His mentor and friend John Stevens Henslow, mineralogist and professor of botany, introduced the young Darwin to both disciplines. Darwin attended Henslow's botany lectures and field trips each year during his three years at Cambridge, visiting also private meetings at Henslow's home. Here he met with Adam Sedgwick, president of the newly formed Geological Society of London. During a geological field trip in the summer of 1831 with Sedgwick, Darwin collected and preserved also some plant specimens.

Herbarium sheet by J. S. Henslow with three plants collected by Charles Darwin in 1831 at Barmouth, North Wales. This is the earliest-known herbarium specimen collected by Darwin.

During the five-year-long voyage of the Beagle Darwin collected plants or seeds on the Cape Verde Islands, in Argentina, in Uruguay, in Chile, in Brazil and some of the visited islands, like the Falkland, Galápagos and Cocos islands. As Darwin had limited space on the Beagle, most occupied by rocks and animals, he limited himself to remote or poorly studied localities.

Darwin had prepared several thousand labels in different colors before the voyage to be applied to every dried plant (the labels including species, locality, date and his signature). Wet specimens, conserved in "spirits of wine", were tagged with a small, metallic plate. Henslow, who back in England managed Darwin's collection, however, removed most labels when including Darwin's specimens into the herbarium. Unlike the collected rocks and animals Darwin didn't number the plant specimens, so it seems a bit confusion sneaked into the collection. Another friend of Darwin, botanist Joseph Dalton Hooker, lamented to Darwin that not all notes could be attributed to the preserved plants.

Darwin's plant collection is especially interesting as it includes many species from less visited islands of the Galápagos and the Cocos archipelago. Darwin was intrigued about the relationship of the isolated species found on the islands to the species found on nearby continents. Later Darwin conducted experiments with seeds, showing that some can survive salty water for months and so be dispersed by marine currents. Despite Darwin's plans, he didn't publish the collected plants in “The Voyage of the Beagle” (published in 1839), as a very busy Henslow didn't meet the deadlines for publication.

Darwin collected 756 different species, subspecies or varieties of vascular plants during his five years long voyage around the world, 220 species were new to science. Darwin was especially surprised by the variability displayed by plants. A collected grass species was divided by Henslow into fifteen different varieties! This was an intriguing observation, important for his later formulated theory of evolution, how one species can split over time in various new ones. Also, the relationship of plant species on islands to nearby continents was an important observation. The plants from the Galápagos islands showed, according to Hooker, a remarkable variability between the single islands, however some even more remarkable similarities to species from North America and Brazil. Would a divine creator not be able to create distinct, unique species on remote islands as he pleased? However, if seeds can be dispersed with marine currents and islands be colonized by plants from nearby continents, couldn't they also evolve there in new species?