Theory:

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Charles Darwin
  
Charles Darwin (1809-1882) was a renowned naturalist and philosopher of the 18th century. In 1809, he was born in England. He was captivated by nature while in college because of his connection with Professor J.S.Henslow. The British Admiralty planned a \(5-year \)exploratory expedition around South America on the H.M.S. Beagle (a ship that sailed around the world) at that time.
 
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H.M.S Beagle
 
As Dr Henslow was asked to nominate a young naturalist for the voyage, he nominated Darwin. In the five years (\(1831-1835\)) voyage period, Darwin explored the flora and fauna of many continents and islands.
 
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Map showing the voyage of H.M.S. beagle

He also visited the Galapagos Islands and Pacific islands of volcanic origin called "a living laboratory of evolution."
 
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Galapagos islands
  
Darwin thoroughly recorded the natural history of the land, plants, and animals in the locations he visited. He spent \(20\)years and developed the theory of natural selection.
 
In \(1859\), Darwin published his observations and results under the title "Origin of Species." Darwin's book proves the existence of evolution. It elaborates on the hypothesis of Natural Selection as a mechanism for evolutionary change.
The research he conducted during this voyage changed the way we think about the diversity of life on Earth forever. Surprisingly, once he returned to England, he never left the country again. He stayed at home and undertook a series of experiments that led him to the conclusion that evolution occurred as a result of natural selection. He was oblivious to the method through which variations in the species evolved. Mendel's experiments would have illuminated him, but these two gentlemen were completely unaware of each other and their work!

Darwin is frequently associated only with the theory of evolution. He was, however, a skilled naturalist, and one of the studies he undertook focused on the importance of earthworms in soil fertility.
Reference:
https://commons.wikimedia.org/wiki/File:Charles_Darwin,_English_naturlist,_colored.jpg
https://commons.wikimedia.org/wiki/File:Voyage_of_the_Beagle.jpg
https://www.flickr.com/photos/nunataak/3348740075