John Tweedie and Charles Darwin in Buenos Aires.

The journey of exploration undertaken by Charles Darwin FRS during the voyage of HMS Beagle has a central place within the historical development of evolutionary theory and has been intensively studied. Despite this, new facts continue to emerge about some of the details of Darwin's activities. Drawing on recently published Darwin material and unpublished letters in the archives of the Royal Botanic Gardens, Kew, we document a hitherto unexamined link between Darwin and John Tweedie (1775-1862), a relatively obscure Scottish gardener turned South American plant collector. All of the available evidence points to a meeting between the two men in Buenos Aires in 1832. Tweedie provided Darwin with information about the geography of the Rio Paraná, including the locality of fossilized wood eroding from the river bank. It also seems likely that Tweedie supplied Darwin with seeds that he later shipped back to John Stevens Henslow in Cambridge. Although this brief meeting was at the time relatively unimportant to either man, echoes of that encounter have resonated with Tweedie's descendants to the present day and have formed the basis for a family story about a written correspondence between Darwin and Tweedie. Local information supplied to Darwin by residents such as Tweedie was clearly important and deserves further attention.

Levels of selection in Darwin's Origin of Species.

References in Darwin's Origin of Species to competition between units of selection at and above the level of individual organisms are enumerated. In many cases these references clearly speak of natural selection and do not support the view that Darwin thought selection only occurred at the level of the individual organism. Darwin did see organismal selection as the main process by which varieties were created but he also espoused what is here termed community and varietal selection. He saw no essential difference between varieties and species and the references show that he also believed that selection could operate at the species level.