"One general law, leading to the advancment of all organic beings, namely, multiply, vary, let the strongest live nd the weakest die."
-The Origin of Species
Charles Darwin's discovery was and still is controversial even though he found evidence of evolution because there are still people who do not believe in it.
In 1931 Charles Darwin embarked on a five year voyage on the H.M.S. Beagle to survey the South American coast under Captain Robert FitzRoy. For the majority of the voyage Charles Darwin was ill while on board the ship but while on land he studied the environment and kept recorded his findings in a journal.
While on the voyage he observed the basic law of nature "kill or be killed" in action. He witnessed animals fighting for survival and came to the conclusion that only the strongest would survive. He also found the remains of long dead species and noted the similarities between organisms both living and dead. Like the finches on the Galapagos islands that were identical except for the shape of their beak, which was specialized for food on different parts of the island. After the 5 year voyage was over he theorized that the organisms had adapted over time from common ancestors to fit better fit in their environment. Darwin was not the first to come up with the theory of evolution, but he was the first to propose an answer to the question how and why organisms evolve over time. The answer was Natural Selection, and that meant organisms with desirable traits for survival lived long enough to reproduce and pass on those traits while organisms with undesirable traits die young and can't pass on their genetic material.