Darwin´s first botanizing steps followed the 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."
Darwin in a letter to Henslow, 1836
 

Darwin´s interests in the natural world were widespread. He enjoyed hunting, later also taxidermy. With his cousin William Darwin Fox he hunted for bugs. He collected rocks and minerals and later geologized around the world during the voyage of the Beagle (1831-1836). His mentor and friend John Stevens Henslow was professor of Mineralogy and later for Botany, introducing the student Darwin in both disciplines. Darwin attended Henslow´s botany lectures, labs and field trips each year during his three years at Cambridge, visiting also private science meetings at Henslow´s home. During the geological field trip in summer of 1831 with Adam Sedgwick he also collected and preserved some plant specimens.

Fig.1. 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 (image source).
 

During the voyage of the Beagle Darwin collected plants or seeds on the Cape Verde Islands (the first stop of the Beagle), then Argentina, Uruguay, Chile, Brazil, later also on 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 concentrated on remote or less well studied localities.
Darwin had prepared several thousand labels in different colors before the voyage to be applied to every dried plant (the labels including plant name, locality, date and his signature). Wet specimens, conserved in "spirits of wine", were tagged with a metal tag. Henslow, who back in England managed Darwin´s collection, removed however most labels when putting Darwin´s specimens into the herbarium. Unlike the collected rocks and animals Darwin didn´t number the plants, 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 plant specimens.
 

Darwin´s plant collection is especially interesting as it includes many species from the – at the time – less visited islands of Galápagos and Cocos islands. Darwin was intrigued about the relationship of the island species to nearby continents, he will do some experiments with seeds showing that some can survive salt water for months and so be dispersed over the sea. Despite Darwin´s first plans he didn't publish the collected plants in his description of “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 voyage around the world, 220 species were new to science. Darwin was especially surprised by the variability of plants, one collected grass species was divided by Henslow into 15 different groups! This was an intriguing observation, important for his later theory of evolution, as variability is where natural selection acts on. 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 could disperse with marine currents and islands be colonized by plants from nearby continents, couldn't they also evolve there in new species?
 

References:
 

PORTER, D.M. (2010): Darwin: the Botanist on the Beagle. Proceedings of the California Academy of Sciences, Vol.61(4): 117-156