The struggle for existence is the filter through which natural selection flows. Since competition for survival between species is fierce, any advantageous variation that a species possesses—wings that allow it to fly, special coloring that allows it to hide from predators, and so on—will give it an edge in the struggle for existence. Organisms with the most advantageous variations, therefore, are most likely to survive in nature. Moreover, their survival over others makes it more likely that they will produce more offspring that carry the same heritable advantageous variations. Since that parent was able to survive over and birth more offspring than others in its species, subsequent generations of that species will see this advantageous trait appear more often.

Eventually, Darwin argues, the perpetuation of this trait will result in an entirely different species from the original, an outcome he calls “descent with modification.” Darwin uses the phrase “natural selection” to describe nature’s process of selecting organisms with advantageous variations for survival and allowing those traits to be perpetuated in subsequent generations.

Darwin does admit that there are limits to the extent to which natural selection can bring about variation in a species. Since the origins of variations are unknown, natural selection can only work with variations that randomly appear. Over time, new variations will appear that are better or more complex than old ones. These new variations will come to replace the old ones, slowly leading to the seemingly perfect adaptive states that are apparent in species existing today.

Moreover, natural selection cannot perpetuate variations that will harm one species solely for the benefit of others. A trait that is harmful to one species but helpful to another—for example, the slow movement of an organism that allows its predators to hunt it easily—cannot be naturally selected solely due to its helpfulness to the latter species. If a species does exhibit a trait that proves beneficial to another, there must also exist some advantageous trait in the first species that allows it to procreate successfully. Otherwise, the first species will eventually become extinct as a result of its harmful variation.

Finally, natural selection leading to descent with modification is understood by Darwin to be slow, gradual processes. Advantageous traits are not selected overnight; it takes many generations before a population slowly begins to diverge from its parent species and many more generations until one trait becomes dominant and another one becomes extinct. Over time, natural selection has led to the successful adaptations of today’s existing species, all of which exhibit outstanding adaptations to their natural environments, and will continue to influence descent with modification as future variations arise and environments change.