Why evolution is a theory?

The theory of evolution was developed by Charles Darwin, an amateur English naturalist, in the 19th century. He proposed that all of the millions of species of organisms present today, including humans, evolved slowly over billions of years, from a common ancestor by way of natural selection. This idea said that the individuals best adapted to their habitat passed on their traits to their offspring. Over time these advantageous qualities accumulated and transformed the individual into a species entirely different from its ancestors. Darwin's general theory presumes the development of life from non-life and stresses a purely naturalistic (undirected) "descent with modification". That is, complex creatures evolve from more simplistic ancestors naturally over time. In a nutshell, as random genetic mutations occur within an organism's genetic code, the beneficial mutations are preserved because they aid survival - a process known as "natural selection." These beneficial mutations are passed on to the next generation.