1850 Millard Fillmore becomes the thirteenth president of the United States, when Zachary Taylor dies in office.
  Albrecht von Kölliker publishes the first textbook of histology, Handbuch der Gewebelehre.
1853 Commodore Perry's "black ships" land in Japan.

Franklin Pierce becomes fourteenth president of the United States.

  Alfred Russel Wallace publishes "On the Law which has Regulated the Introduction of New Species," anticipating Darwin's theory of evolution by natural selection.

Rudolf Virchow states the principle that new cells come into being only by division of previously existing cells: "Omnis cellula e cellula."

  Gregor Mendel, a monk at the Augustinian monastery of St. Thomas in Brünn, Austria (now Brno, Czechoslovakia), begins breeding experiments with the garden pea, Pisum sativum.
1857 James Buchanan becomes fifteenth president of the United States.
  Alfred Russel Wallace sends to Darwin a manuscript - "On the Tendency of Varieties to Depart Indefinitely from the Original Type" - that shows clearly that Wallace has independently formulated a model of evolution by natural selection.

Darwin's and Wallace's ideas are jointly presented to the Linnaean Society of London.

  Darwin publishes On the Origin of Species.