"Laws of Variation" is the fifth chapter of On the Origin of Species by Means of Natural Selection by Charles Darwin. In this writeup, though, I concentrate on the version of this chapter presented in Darwin's Ghost by Steve Jones. This is because this chapter of the book deals with the science of genetics, which was not at all well developed in Darwin’s time. As Jones writes in his book, "This chapter is less faithful to the original than are others, if only because Darwin got it so wrong." Darwin realized that he and his contemporaries had little understanding of inheritance, and wrote in his summary at the end of the chapter, "Our ignorance of the laws of variation is profound."
Jones starts this chapter by dealing with the "notion of inheritance of acquired characters," the idea that something that changes about a plant or animal during its lifetime can be inherited by its offspring. Genetically, it can’t. This was not fully understood in Darwin’s time, but it is essential to his theory. If children can inherit genetic attributes developed by their parents during a single lifetime, there is no need for another mechanism, such as natural selection, for evolution. As research and common sense have shown, people and animals do not pass on physical changes genetically to their young. A woman who loses her hand in an accident does not give birth to one handed children.
However, as Jones writes, "Parents and their young nearly always share environments as much as they do genes." Besides their genes, parents typically pass on all sorts of habits and knowledge to their children. Nutrients and chemicals are also passed on from mother to child. Although what happens during their lifetime does not affect the genes a parent passes on, it may affect the circumstances in which their child lives his life.
Jones also tells us that the foundations of the studies of inheritance and genetics come from "a single ugly fact, the reappearance of a character in a lineage after it had been lost." By this he means that recessive traits are the only thing that separates us from a simple, easy-to-understand version of genetics. "It was a very surprising fact," Jones writes, "that characters should appear after having been lost for many, perhaps for hundreds of generations." He describes Mendel’s experiments with peas, and notes that "Mendel saw the value of his work to the theory of evolution," but biologists who knew of his research considered it to be of relevance only to peas.
The lack of understanding of genetics in Darwin’s time is especially highlighted when Jones answers some questions posed by Darwin in The Origin of Species. For instance, Darwin asks, "What can be more singular than the relation between blue eyes and deafness in cats?" Jones explains that "[b]lue-eyed cats are deaf because those at first sight unrelated characters share an embryonic pathway. The gene involved makes the dark pigment—melanin—that is responsible for skin color." Melanin, as researchers have found, is used by the body in other ways besides skin color. One of these is eye color; cats without melanin have blue eyes. Jones continues, "Melanin plays an unexpected part in the brain, for it guides the cells responsible for certain nerve pathways to their correct places. As a result, a shortage of melanin gives a cat a white coat and blue eyes—and makes it deaf." Darwin did not have this kind of detailed biological information, so he was not able to do a good job of explaining how certain traits turn up together.
In addition to all the scientific arguments that both books deal with, though, there are philosophical arguments for evolution. At the end of this chapter Darwin explains one of these, as he describes the Creationist idea that species were independently created, but with some tendency to vary. "To admit this view," he writes, "is, as it seems to me, to reject a real for an unreal, or at least an unknown, cause. It makes the works of God a mere mockery and deception; I would almost as soon believe with the old and ignorant cosmogonists, that fossil shells had never lived, but had been created in stone so as to mock the shells now living on the sea-shore."
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