Darwin uses several examples in his argument of evolution through natural selection. Some of his arguments, which he lists on the last page (513) include growth with reproduction, inheritance, variability, ratio of increase, struggle for life, divergence of character and extinction. Through time, even with little evidence from the geological record, it is clear that life is constantly changing. Darwin tries to prove this by showing the readers that these changes are so small and minute that they are hardly discernable. Just because we can’t see these changes doesn’t mean they aren’t happening. He discusses that competition between species as a driving force for these changes. He also discusses why there might not be fossil evidence of changes or variations between species. Even though we can gain a lot from looking at the geologic record, there are certain aspects of life that just haven’t been preserved. I like how Darwin mentions at the end that we shouldn’t use organic change within the geologic record to measure time. These changes could have happened rapidly because of environmental changes or new competition between species.
To me, the most convincing part of his arguments was variability. Species that underwent extreme migration or environmental changes seem to have the most variability. He talks about how species of the Galapagos islands have a striking resemblance to the animals and plants found on the American mainland and the same for Cape de Verde and the African mainland. Different foreign competitors and differing environments ultimately is what helped these species live and thrive and evolve into what they have become today. “It is a rule of high generality that the inhabitants of each area are related to the inhabitants of the nearest source whence immigrants might have been derived.” (Pg. 499) I also really liked that Darwin talks about man’s role in variability and that we do not directly produce it but unintentionally we can expose plants and animals to new conditions, which in turn might force them to change.