Darwin returns to his theory of descent with modification to clarify the details of how natural selection works to create new species. He stresses that species formation is a gradual process. As modifications slowly develop over time, new species come to replace old ones, causing the extinction of older, less developed species. Some species modify more quickly than others, but all species go through modifications leading to the descent of new species. New modifications lead to more complex and perfectly developed species, and over time, numerous species form, creating chains of descent that form a pattern connecting old species to new, all through the mechanism of natural selection.
In this scheme of natural selection, the extinction of some species is inevitable. Darwin finds the process of extinction mysterious, because he cannot tell what types of “unfavorable conditions” lead to the demise of particular species. He can only surmise that natural selection and the struggle for existence dictate which species can and cannot survive. For example, species that are similar usually provide the strongest competition for one another. If two similar species were in competition with one another, one species would most likely prevail, while the other would become rare in nature and eventually die out. Darwin explains that it is highly unlikely that once a modification has become extinct, the original parent species could produce the same variation again and have it survive in a potentially similar environment. Moreover, it is possible that the original parent species would have already become extinct, rendering additional descent from that parent impossible. Extinction serves as a reminder of the precarious existence of species in nature and the power of natural selection.
Athhough natural selection acts only on individual species rather than on entire ecosystems (or biological communities) at once, Darwin argues that changes in species throughout the world can occur simultaneously. Geological evidence shows how changes in the earth’s surface have occurred similarly in various parts of the world. For example, similar “chalks” (limestone formations) have been discovered in Europe, North America, and South America, and similar fossils have been found in these similar chalks. Darwin concludes that landforms and species can develop at the same time in separate locations, a process he calls “parallel succession” and attributes to parallel geological environments. However, he argues that species do not remain in these separate locations forever—they can migrate, and the introduction of new species into an environment causes new competition that can once again drive the process of natural selection. Natural selection and individual species formation are both processes of independence and interdependence: Species can form separately and simultaneously, but the formation or migration of one species can also affect the formation or survival of another one.
Darwin reviews how the process of descent with modification creates “affinities,” or relationships, between different species. Here, he refers back to Chapter IV, which contains a chart resembling a family tree. The process of modification shows how species are “related” to one another, forming “species families” in which one parent species gives birth to numerous other species. The fossil record is also helpful in discovering which species relate to one another, which may not be apparent from their external appearance. Two species, though related, may be more similar to their common ancestor than they are to each other.
As more species form, however, they become more complex in comparison to their original species. Descent with modification always leads to greater complexity and overall improvement in species that survive and proliferate. Certainly, not all species that exist today are in their most highly developed forms. In fact, Darwin notes that “feebler” forms do still exist, as species develop only as much as their environmental needs and availability of variations allow them to. He explicitly applies this hierarchy to species from different countries, noting that species of Great Britain, which can survive in many different environments, are more highly developed than the species of New Zealand, which would not be able to survive in environments outside their own. Similarly, fossils in Australia that resemble existing species in South America show the South American species to be on a lower evolutionary level, as parallel species in other countries have already become extinct. Darwin concludes that over time, with changes to geological conditions and the introduction of new species through migration, less complex species forms will die out, leaving more complex and highly developed species.
In Chapter X, Darwin again personifies nature as a source of mystery and wonder. He admits that he cannot explain elements of his own theory of natural selection, even while drawing scientific conclusions based on observation and experimentation. He discusses extinction as an inevitable result of hostile environmental conditions, yet says he does not know exactly what conditions and variations cause particular species to become extinct—though he does hypothesize in the text. Although many aspects of natural selection remain a mystery, the very fact that the process occurs strikes Darwin as evidence of the wonder of nature.