What were the views of Charles Darwin about the evolution of life forms?

Explanation

Views of Charles Darwin
The existing living organisms share similarities to varying degrees, not only among themselves but also with the life forms that existed millions of years ago.
There has been a gradual evolution of life forms during different periods of geological history. Extinctions also have occurred.
Any populations have a built in variation for characteristics. Individuals with those characteristics which enable them to survive better (fitness of individual) in natural conditions, would out number others which are less adapted under the same natural conditions.
Such a fitness of the individual refers ultimately and only to reproductive fitness.
Such fit individuals leave behind more progeny than others and are selected by nature to survive and reproduce to increase their population size.