Darwin used a vast collection of information and ideas to convince people of natural selection. Some of those reasons include: the similarity between species, species from the same continent will be more related, domestication, more dominant groups beat less dominant groups, common animals found on islands, similarity of structures within creatures and why animals have traits they don’t need. Darwin explains that two similar species will have a link, but most scientists class it as a distinct species. (pg. 486/487) Species that are found on the same continent will be more related because they will have shared the same progenitors. (pg. 498) The domestication of animals shows how certain desired traits were selected (maybe unintentionally) because humans kept and bred those animals with those traits. (pg. 488/489) Groups of species give birth to new forms so that groups become exceedingly large but more dominate groups will be able to outcompete less dominate groups to keep populations in balance. (pg. 492) Animals that are not capable of crossing wide areas of ocean are not found on islands but winged animals are. (pg. 499) The internal structures of creatures are found to be similar is species that do not look alike at all; for example, the hand of a man and the wing of a bat. (pg. 499) Lastly, if animals were created perfect then why do some possess traits they do not need; for example, “upland geese, which never or rarely swim, should have been created with webbed feet.” (pg. 492/493)
I think the most convincing evidence is the similarity in the internal structure of organisms. This shows how a huge variety of animals that live on land, in water or are capable of flight have related traits. These traits most likely came from a common ancestor, showing that “accumulating slight, successive, favorable variation” over a very long period of time can produce large changes in structures. Even though I like the argument about why animals have traits they do not need, I think it is a weaker argument. As this process can create some unusual characteristics it could be argued that those traits are in fact necessary.