"Geographical Distribution" is the eleventh chapter of On the Origin of Species by Means of Natural Selection by Charles Darwin and Darwin's Ghost by Steve Jones. The twelfth chapter of each of these books in entitled “Geographical Distribution—continued.” In these chapters, Darwin and Jones look at how species have moved around the Earth, and how this movement has affected their evolution. In less historical terms, they talk about why species are where they are now.
Darwin begins chapter eleven by noting that "the first great fact that strikes us is, that neither the similarity nor the dissimilarity of the inhabitants of various regions can be accounted for by their climatal and other physical conditions." For instance, "if we compare large tracts of land in Australia, South Africa, and western South America," Darwin writes, "we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar."
The reason for this difference is that there are obstacles to the free migration of species. Animals from Australia are not found in South America because they cannot get there, not because they wouldn’t prosper if they could. And oceans are not the only barriers to migration. Mountains are also tremendous obstacles for many species, as are areas of different climate dividing two areas of similar climate. For sea animals, land is a barrier. The sea life on the east coast of Central America is very different from that on the west coast, even though all that separates them is a few dozen miles of Panama.
But even with all of these obstacles to migration, we see similar species existing in different parts of the world. Darwin continues by writing that "the several species of the same genus, must originally have proceeded from the same source, as they are descended from the same progenitor." Even though we don’t find the same species in different parts of the world, we do find related ones. These related species must have either migrated to where we find them now or be descended from other species that made the migration.
This migration between places now separated by water and mountains would sound unlikely to someone who had not heard geological ideas about the movement of continents and the immense changes the Earth constantly goes through. But, as we now know, climates and barriers change. "At the end of the last ice age, just eighteen thousand years ago," Jones writes, "there were polar bears in the South of France, and even today the Mediterranean has its own whales and seals." The polar bears got to France over glaciers, and the Mediterranean was at one time open to the Atlantic Ocean.
The continents have moved and separated over time as well. "Once," Jones notes, "there was talk of land bridges between the continents, used as highways for trees, dinosaurs and more on their global journeys.… So many links between distant places were needed to explain the distribution of animals and plants that the seas were filled with theoretical Atlantises.… The truth is more remarkable, for the land itself, rather than the sea, is on the move."
In Darwin’s time, people even questioned "whether species have been created at one or more points of the earth’s surface." Either each individual species has migrated from a single point of conception, or the same species have evolved multiple times at different points on the Earth. The first of the hypotheses clearly makes more sense to Darwin’s theories, but Darwin admits that there are some cases in which it is hard to understand how creatures could have migrated. Even so, he has a strong argument for a single original environment for each species: "the simplicity of the view that each species was first produced within a single region captivates the mind," he writes. "He who rejects it… calls in the agency of miracle.” And, with the best evidence yet gathered, neither a miracle in the form of land bridges nor a miracle in the form of repeated generation of the same species is necessary to explain the migration of species. Jones ends his eleventh chapter on a wondrous note. "Land bridges, Atlantises, and all the other myths dreamed up to explain the distribution of plants and animals are less remarkable that the truth: that the world has evolved as much as have its inhabitants."
Darwin apparently did not think that one chapter on the distribution of species was enough, however, so he wrote a second one, entitled "Geographical Distribution—continued." In following Darwin’s chapter structure, Jones did likewise. In The Origin of Species, this chapter begins with the observation that, since lakes and rivers are separated from each other by land, "it might be thought that fresh-water productions would not have ranged widely within the same country." Surprisingly, "the case is exactly the reverse": many fresh water species can be found in numerous bodies of water spread out over great distances. There are a few reasons for this, the most important being that rivers change directions over time, and as they do so, they may meet up with other rivers and lakes, only to separate later. When these rivers meet up, fish from one river can swim into the other, which creates competition between the environments. When rivers or lakes separate, each is left with a similar set of species.
Jones also explores how species migrate into a specific, somewhat closed-off area, with the example of Krakatau, a volcanic island in Indonesia about thirty miles from Java. Before 1883, Krakatau was heavily forested. On August 27th of that year, the volcano erupted, causing a tsunami that drowned over thirty thousand people and spewing dust that literally travelled around the world. In New York State, firefighters were called out "to fight what seemed to be a giant blaze in the distance." A week later, two-thirds of the island and nearly all of its life was gone. The first visitor after the eruption found only a single spider. Over time, though, life returned. By the turn of the century, "grasses and cane to the height of a man covered Krakatau." In addition, many birds and lizards have come to the island. Even so, it is still sharply different from nearby Java: there are no monkeys, cats, or frogs on Krakatau. Otters are the only land mammals. Many other islands have similarly strange mixtures of inhabitants, but Krakatau is unique in that humans were able to observe its changes; its death and its rebirth.
Darwin wrote The Origin of Species before Krakatau erupted, but he still knew much about oceanic islands. He uses some of his observations to make another argument for the theories of evolution. He notes that many islands, such as Ascension, St. Helena, and New Zealand, once had very few plants. Ascension, he writes, "aboriginally possessed under half-a-dozen flowering plants." Despite this poverty of flora, these same islands have been excellent environments for other plants brought in by humans, and in some cases these exotic plants have exterminated the native ones. And now for Darwin’s point: "[h]e who admits the doctrine of the creation of each separate species, will have to admit, that a sufficient number of the best adapted plants and animals have not been created on oceanic islands; for man has unintentionally stocked them from various sources far more fully and perfectly than nature." In other words, if God created individual species and decided where to place each, why did he put the weak ones on the islands? And why so few?
Coming back to the animal inhabitants of these islands, Darwin notes frogs, toads, and newts are never found as native species on oceanic islands, but are very successful when introduced to the islands. These same species are "immediately killed by sea-water," as are their eggs, so it is impossible for these animals to travel to an island. But, Darwin adds, if one believes in creation of individual species, why "they should not have been created there, it would be very difficult to explain."
These same remote islands are frequently inhabited by bats and no other mammals. Why? Because bats are the only mammals that can fly there. Indeed, bats are known to travel distances of over 600 miles over water, from North America to Bermuda.
Before finishing with the topic of islands, Jones has one more interesting observation. Because many islands have no trees, but only small plants, plants tend to evolve larger on these islands in order to compete with other small plants for sunlight. This eventually leads to tree-sized varieties of species that are typically much smaller, such as daisies, lettuce, cabbage, and even celery.
This illustrates another of Darwin’s statements in this chapter. He writes that there is a "deep seated error of considering the physical conditions of a country as the most important for its inhabitants; whereas it cannot, I think, be disputed that the nature of the other inhabitants, with which each has to compete, is at least as important, and generally a far more important element of success." Indeed, celery plants grow to only a small height, no matter what their physical environment, as long as there are trees around, but without trees, they are able to achieve much greater heights.
Darwin ends his second chapter on "Geographical Distribution" by writing that the facts and observations discussed in this this chapter are "utterly inexplicable on the ordinary view of the independent creation of each species, but are explicable on the view of the colonization from the nearest and readiest source, together with the subsequent modification and better adaptation of the colonists to their new homes."
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