Darwin decided it was possible for species to change from one form and develop into another over time.  This led him to the notion that all life forms were not fixed but continuously changing or evolving.   The other part of the theory was that living things weren’t the result of many separate creations (as had been suggested in Genesis) but of long, intertwining biological histories.  His general idea was that amongst a family of plants or animals individual members carried hereditary traits.  These traits would be general to the individual’s family, but not the species. It is these traits which could give the member a better chance of survival and reproduction.  This is what Darwin called Survival of the Fittest  or Natural Selection.  Those individuals with slightly better adaptations, according to the theory, would get more food, be healthier, live longer and, most importantly, have more mates.  As time progresses, traits become more obvious, therefore later generations will be more defined and, possibly after thousands of generations, form a new species. 

An example of this theory can be seen in the Galápagos Islands.  The Galápagaos are 16 islands off the coast of South America that Charles observed in 1835.  These islands, we now know, were formed 4 million years ago from volcanic activity and had no life on them.  Therefore, any animal living there now must have either flown or drifted to the islands.  One species thriving in the Galapagos is the Marine Iguana, the only sea going Iguana (a type of lizard) in the world. So, if it is known that the Galápagos were initially empty, “Where did this Iguana come from?” Darwin asked.  Other reptiles came to the Galapagos Islands like the Yellow Land Iguana from South America. Reptiles such as these, being able to survive for long periods of time without food or water, could have drifted to the Galápagos on driftwood or bark.  Suppose the land Iguana had no food, due to overpopulation, and was forced to feed in the shallow waters on seaweed, small fish and molluscs.  If one Iguana could swim better and could hold onto rocks better than others, even slightly, it would get more food and become stronger and healthier.  It would therefore win more mates and reproduce more whilst its other brothers and sisters die from starvation.  Later generations from this Iguana inherit and refine this talent and will survive and reproduce.  Now, there are more of these skilled Iguanas who can also gather food from the sea. Thousands of generations later, with these traits selected, we end up with a new species, the Marine Iguana, which now far outnumbers the Land Iguana. 

This is only a suggestion of how the Marine Iguana has evolved, but if Darwin’s "Origin of Species" stands, it is a realistic one. 

As you can see, there are two vital steps to Darwin’s theory the first of which is heredity.