Darwin's voyage on the H.M.S Beagle (the name of his ship) took him past Australia, the tip of Africa, to Spain, back around South America. One of the most significant observations that Charles Darwin made was that there were varieties of similar animals on continents that were located on opposite sides of the world from one another.
At one time, all the continents were thought to be connected in one large land mass known as Pangea. Darwin reasoned that these organisms came from a common ancestor that lived on this landmass. When it separated and the continents drifted apart from one another, the separated populations were forced to adapt to their new habitats. Natural selection is the process in which the environmental elements "select" which organisms will survive. The varieties of an organism that have slightly advantageous characteristics will survive longer, be more likely to reproduce, and therefore, pass on their genes to future offspring in that area. For example, a white bear will be more likely to survive the North Pole (due to being camouflaged) than a brown bear. Theoretically, bears would have less reproductive success to due predation, and their genes would be removed from that area's gene pool over time.