Darwin argues his theory through several means. He addresses the geographical distribution of species, the lack of “intermediate varieties”, the disuse of certain biological features by some animals, and he ties these together to explain why species aren’t “well-marked and permanent”. Darwin explains the geographical distribution of species through “much migration” throughout the ages, and that the changes in climate and geography drive changes in species. When addressing the lack of intermediate forms, Darwin points to the geological record, and states, “If we admit the geological record is imperfect in an extreme degree, then such facts as the record gives, support the theory of descent with modification.” (Darwin, 496) He explains that while the fossil record may lack direct, successive intermediates, it shows the connection between modern and ancient species. To illustrate his point about disuse of features, Darwin uses the example of a calf, which possesses teeth that will never “cut through the gums of the upper jaw, from an early progenitor having well-developed teeth,” and states, “we may believe, that the teeth in the mature animal were reduced, during successive generations.” (Darwin, 502) All of these points come together to help support Darwin’s theory of descent with modification by way of natural selection, and they help demonstrate the idea that species are not, in-fact, well-marked and perfect.
I found Darwin’s arguments about intermediate forms and the disuse of features to be the most convincing. Darwin admits, that while we don’t see any “intermediate forms” of species roaming the planet, and we lack some in the fossil record, we can trace and make comparisons with ancient fossils and modern species. He states, “Thus we can see why the more ancient a fossil is, the oftener it stands in some degree intermediate between existing and allied groups.” (Darwin, 497) Darwin’s argument that we can use ancient fossils to draw connections between certain modern species helps demonstrate the idea of common ancestry well. In regards to the disuse of certain features, we see many animals today with such features. Darwin’s calf teeth example is one, but there are others instances such as leg bones in whales and dolphins or the human appendix. As Darwin says, species are not perfect. The evidence we have gathered wholeheartedly supports Darwin’s theory, and he provides an excellent summary of this evidence. His closing statement, in which we are placed on an entangled bank to imagine the lifeforms that inhabit it, truly provides a beautiful picture of evolution. “There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” (Darwin, 513)