"I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection"

Charles Darwin's quote is describing the procedure of natural choice, which is an essential principle of evolutionary biology. Natural selection is the procedure by which organisms with beneficial traits are most likely to make it through and reproduce, while those with less useful traits are less likely to make it through and reproduce. This process is driven by the environment, and the environment chooses for qualities that are beneficial in that particular environment. Darwin's quote is emphasizing that even small variations in traits can be beneficial and can be maintained through natural selection. This is an important principle in evolutionary biology, as it discusses how species can adapt to their environment and how brand-new species can arise. Natural choice is an effective force in nature, and it is accountable for the variety of life that we see today.