"Variation Under Nature" is the second chapter of On the Origin of Species by Means of Natural Selection by Charles Darwin. It is also the title of the second chapter of Darwin's Ghost by Steve Jones. Both Darwin and Jones use this chapter to examine the degree of variation in nature. They also look at the uses of words like species, subspecies, and variety.
The primary question explored in these chapters is about the use of the word species. What is a species, exactly? As Darwin wrote, "No one definition has yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species." We know, for example, that two animals of the same species are not exactly the same. One can tell this from looking at two people, or examining the DNA of two mice. Generally, though, members of the same species are mostly similar.
How similar they seem, though, may depend on how closely we look. As Darwin writes, "It should be remembered that systematists are far from pleased at finding variability in important characters, and there are not many men who will laboriously examine internal and external organs, and compare them in many specimens of the same species."
Jones agrees. "[T]o share a Latin name imposes, almost by definition, a certain uniformity upon those who bear it," he writes. "That comforts both creationists and experts on taxonomy. They like to see existence as a set of neat ideals, each filled with some pure Platonic essence. However, a great deal is hidden within even the most uniform creature."
Many animals, such as fruit flies, appear identical. This appearance can hide genetic differences, though. Occasionally, what is thought to be a species is found to consist of multiple groups of animals that do not interbreed. An example is what is known as the leopard frog. As Jones writes, "[W]hat was once classified as a species is now thought to be at least twenty-seven, different in their DNA and quite unable to cross."
The many species being discovered today, such as these leopard frogs, make up one of the best pieces of evidence against Young-Earth Creationism. As Jones explains, biblical scholars estimate the length of Noah’s Ark at 450 feet. "Nowadays," he writes, "it would have to be a lot bigger. As well as three hundred and thirty thousand kinds of beetle, the Ark might—depending on its captain’s views on classification—have to accept the half-dozen varieties of tiger and twice as many leopards." In other words, if one accepts the Bible as literally true, either Noah’s Ark was very, very big, and biblical scholars are wrong about its size, or there are many fewer species than we think, and biologists are wrong, or new species have evolved since the Flood. While each of these hypotheses explains the conditions of the world, the last one does so better than the other two, as it integrates and accepts knowledge from both fields.
Of course, this decision is relevant only if one accepts the Bible as literally true. If one does not, there are many more explanations for the existence of so many species. From observation and study, biologists have been led to the conclusion that evolution by natural selection successfully explains the number and diversity of species in the world.
Later in each version of the chapter, proposals are made for the definition of a species. Darwin first clearly states that species do not have "some pure Platonic essence." He continues, "I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other."
At the end of his version of the chapter, Jones provides a different definition of a species, this one based on the future. He argues that animals are of the same species if they will someday share a descendent. "Museums assume," he writes, "(and it seems fair) that cats and dogs are separate because there will never be an animal that traces a shared descent from dogs and cats. This assumption is impossible to test."
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