Darwin’s theory of natural selection came to his mind when he was done travelling the world. And he saw that species vary from one place to another. Then he said to himself, why is that? Why do some work over here and not over there? Why do some live in here and not in there? And then he thought about how if we did not like something we will get rid of it and get something else to replace it. For example, let’s talk about cows. The cows that we humans like keep to get milk from, however, the cows that we don’t like would become hamburger. So he said if we can do it, why can not nature do it. So he came out the the idea of natural selection.

Darwin defined his idea of natural selection with Genetic organisms will undergo genetic change over time due to natural selection. And he added, if enough changes occur over time (centuries) New species would develop from pre-existing ones .

So we can say that natural selection is those with a genetic trait that increases the chance of offspring will pass their genes on to the next generation more than those without it.

Modern science suggests that there is even something called Genetic Drift, which basically means random changes in small groups
Darwin used some evidence to back him up. Such as Fossils, drawn saw fossils of creatures that are not present today when he was in his journey. So he said these species are gone but they share great semblance to modern species.
Embryology, species that have similar developments from when the egg is fertilized to when they grow up i mean in the embryo stages are more closely related.
Comparative anatomy, lets say when you look at your hands and compare them to the hands of the bats, you can see that they are closely related. But if you look at your hand and a wing of a bee, they are similar.
Molecular Biology, it is a new tool that helps you compare the genes them selves.

Darwin’s theory makes more sense than the Lamarck’s theory. Lamarck’s theory was that the giraffes made their necks grow longer by having one generation stretch then passing that stretch to other generations, and so on so forth that it become the giraffe that we know of today.