There are a couple of important points Darwin makes when considering his own argument of evolution through natural selection. Things like geographic distribution, the impact of man, and the types or heritable traits. Geographic distribution is something that plays a huge role in his theory of evolution. Two groups of the same species are separated by something like the mountain or the ocean. Over time these two species will develop and change the heritable traits that they pass on to their offspring. “The existence of closely allied or representative species in any two areas, implies, on the theory of descent with modification.” (Darwin 499) They need to do this to survive in their direct environment. Eventually over time both these groups of the same species will begin to resemble completely different species all together due to the traits they need to survive in their own environments. The traits that are passed down of course are the other factor to this equation. Most traits are heritable, meaning that they are passed down from generation to generation. Then these species often produce more offspring than can be sustained by their environment. They fight to stay alive and certain traits are continually passed down based on who has the most success in staying alive in their environment.
I think the most convincing argument is the relationship between geographical environment and barriers and species adapting to these environments. There are many species that are extremely like one another that live in two separate locations. They generally have adaptations that are needed to survive in the environment that they are living in. This is the part that seems almost irrefutable. These two groups that once possibly lived together have both survived in separate environments due to separate mutations that allowed them to thrive. The ability to change your traits based on surrounding is the best defense of natural selection.