Darwin used a few different arguments to back up his idea of evolution through natural selection. One of the pieces of evidence he used was distribution. He talks about how, if we all agree the geology and geography of the Earth was so different in past history, we must recognize that animals and plants on differing parts of the globe came from similar ancestors. With this belief, we can then look at each individual and see that there are slight differences between them. This also leads somewhat into his idea of the rudimentary organs. “Disuse, aided sometimes by natural selection, will often tend to reduce an organ, when it has become useless by changed habits or under changed conditions of life,” (Quammen 500). Over time, as species don’t use certain organs as much as they used to, the organs will either reduce in size, or like the calf’s teeth, not quite even expose themselves. As seen on an x-ray, they are still there just not accessible. From here, the next sensible piece of evidence Darwin mentions is the bone structure. “The framework of bones being the same in the hand of a man, wing of a bat, fin of a porpoise, and leg of a horse, — the same number of vertebrae forming in the neck of a giraffe and of the elephant,” (499). This is strong evidence of animals coming from similar ancestors but adapting overtime within their own environment.
I think the strongest piece of evidence he suggests is the comparable bone structure. Personally, I find it hard to argue with the same number of bones, similar placement, and related use in each animal. The hadn’t of a man, wing of a bat, and fin of a porpoise can/are all used for propulsion or some sort. I have heard of this before, and to date, this is still one of the most interesting finds to me.