How does Darwin's theory of Natural Selection explain the appearance of new forms of life on earth ?

Explanation

Branching descent and natural selection are the two key concept of Darwinian theory of evolution. The evolution of life forms had occurred but driven by use and disuse of organs . He gave the examples of Giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks . As they passed on this acquired character of elongated neck to succeeding generations, Giraffes, slowly, over the years, come to acquire long necks.
Darwin said that for a pariod of time, over many generations survivors will leave more progeny that have variations which are heritable and make resource utilization better. This will lead to a change population characteristics and hence, new forms appear to arise.