Darwin begins his argument by pointing out that although the geologic record may seem to be provincially lacking evidence to support his theory, if one examines it on a greater scale the changes in fossils demonstrates its accuracy. The Earth has changed over time, he says, and there is no reason why its organic habitants shouldn’t be subject to the same fate. Descent with modification begins with variability within a species, which Darwin points out humans already exploit in domesticated livestock. This variability produces individuals who are better suited to their environment, who will then outcompete other individuals and pass on these desirable traits. Darwin comments that the differences this creates in populations often makes it hard to distinguish the line between varieties and species, but that time and geographical isolation solidify the differences. Looking at the geographical distribution of species offers more proof of his theory; for example, that terrestrial animals are found less on islands far from the mainland but that bats and birds are more prevalent. Island species are usually distinct from their mainland relatives, which is explained when one considers that they evolved in isolation. He also references how the embryonic forms of mammals, reptiles, birds and fish are almost identical, and only become distinct in the later stages of development—which is the result of some ancient common ancestor. Even once they are clearly separate species and even separate classes of species, the “framework of bones [is the same] in the hand of a man, wing of a bat, fin of the porpoise, and leg of the horse” (499). These are all clues that point to descent with modification as the correct evolutionary theory.
The most convincing aspect of this argument is the initial claim that variability within a species exists. This is an easily verifiable claim, whether we observe it in wild animals, household pets, or ourselves. Slight genetic differences cause phenotypical expressions that will be favorable to some organisms and put others at a disadvantage. Humans have profited off of this fact for thousands of years, and have fundamentally changed (and created) species. Perhaps the least convincing argument Darwin offers is the similarities during the fetal stage; although the pictures do show the resemblance, embryonic development is less accessible to the common reader and may hold less weight than something they can actually see with their own eyes.