The theory of natural selection is based on four basic facts:
They, in turn, transmit to their children, which transmit to theirs and so on. Accumulation of such changes in a suitably long period of time can produce surprising results. Natural selection explains an astonishing diversity among living organisms and their perfect adaptation to the environment.
Although Gregor Mendel lived in the same period as Darwin, his work was almost unknown at that time. Darwin noticed the existence of evolution but could not explain its mechanism. He did not know why the organisms were different and how their adoptive traits could be transmitted to their progeny.