On the Origin of Species

Darwin employs this principle to explain how the minute differences of newly propagated varieties transmute the wide variability between incipient species. The most thoroughly diversified animals or plants tend to become even more so because they outcompete their less diverse ones. Divergence, therefore, builds on divergence, leading to even greater variety and the transmutation of species.

Although he states that it’s not a precise definition, Darwin refers to instinct as unconscious actions by animals that would require a learning experience for humans to do. Instincts are generally observable from a young age. The animal, Darwin writes, acts on instinct “without knowing for what purpose” (699).

As the central concept of Darwin’s theory, natural selection is of fundamental importance. He characterizes it as the “preservation of favorable variations and the rejection of injurious variations” (601). Essentially, in the struggle for existence within a complex, competitive network, any minor variation that gives an organism a slight advantage makes that organism more likely to survive, reproduce, and generate offspring with the same variation. Such a variation is thus naturally selected for.

This is the system of mutual affinities among species in the “community of descent.” Whereas some naturalists attempted to classify organisms according to the “plan of the Creator” or, instead, by mere resemblance, Darwin considers the true account of the Natural System a classification according to the genealogical history of branching species.

Taking inspiration from the work of Thomas Malthus, Darwin notes that for any population of a species, more organisms are produced than can survive. “Geometrical increase” refers to what we now call exponential growth. If the conditions for life even slightly tilt in the favor of a specific organism, it propagates at a much higher rate.

This form of selection is based on the competition between males (in most cases) for the interest of the female. The selected males generate offspring, thereby ensuring that their traits are more likely to propagate across generations. Although the result of failure to reproduce isn’t death, as in the struggle for existence, it means that the individual’s line of descent ends.

A problematic but essential term for biological study, species (according to Darwin) generally refers to “the unknown element of a distinct act of creation” (572). “Distinct act” separates the usage of the term “species” from “variety.” Beyond this, no clear distinction exists between the two terms.

This phrase, which Darwin commonly uses throughout his work, refers to the hardships that organisms (both plants and animals) face in their fight for existence. These hardships include the demands of their physical environment, competing with others (both within and outside a species) for scarce resources, and the possibility of mating. Given the supposed “principle of geometrical increase,” Darwin considers it law of nature that “more individuals are produced than can possibly survive” (587), and this is one of the several senses in which Darwin refers to “the struggle for life.”