What are fossils and how do they support Darwin's ideas?

Darwin essentially argued that living things are engaged in a 'struggle for existence' and that any features they possess which affect their survival and reproductive potential will be subjected to continuous selective pressure. This might be seen as his 'new big idea' in the whole evolution thing. This means that through time those features which confer an advantage in the struggle for life will be retained and those that constitute a handicap will be lost. Not for individuals but for the race. So the composition of the population (or species) changes through time, eventually producing such widely divergent forms that cannot interbreed, and are so called separate species. That is a brief outline of the theory.