In the penultimate chapter of _Variation,_ Darwin attempted to strengthen the weak link in his chain of argument by proposing a mechanism for inheritance. He called his theory “pangenesis,” and he claimed that it explained not only ordinary inheritance—the influence of parents on their children—but also reversion, telegony, the regeneration of amputated limbs in some kinds of animals, the inheritance of acquired characteristics, and the relationship between sexual and asexual modes of reproduction and inheritance. The operation of pangenesis depended on the posited existence of unobservable units that Darwin called “gemmules,” tiny granules that were thrown off by individual cells and then circulated through the body. They had, however, an affinity for each other, which led to their aggregation in the reproductive organs or in parthenogenetic buds. They could remain latent for years, until an organism reached a certain stage of development, or for generations, until they encountered other gemmules to which they bore some special relationship. In this way a long-dormant greatgrandparental gemmule might suddenly manifest itself in a child. Since gemmules could he altered by environmental influences, they could convert acquired characteristics into the stuff of heredity. And since they were vulnerable to error, they could occasionally make mistakes, causing organs, such as limbs or tails or even heads, to develop in inappropriate numbers or in the wrong places.
It has doubtless been fortunate for Darwin’s reputation that his theory of pangenesis is not as well remembered as his theory of evolution by natural selection. As vague in detail as it was ambitious and comprehensive in scope, it was unpersuasive at the time and has since been proven completely wrong. But like _Variation_ as a whole, which similarly illustrated the limitations of its author as well as his strengths, pangenesis does not therefore lack interest or significance. Despite recent excellent and well-appreciated studies of his entire life and extended _oeuvre_ (Janet Browne, _Charles Darwin: Voyaging_ [New York: Knopf, 1995] and Adrian Desmond and James Moore, _Darwin_ [London: Michael Joseph, 1991], Darwin is known primarily as the author of the _Origin,_ which is unrepresentative in its economy of structure, argument, and evidence, as well as on account of its historical notoriety. Its enforced streamlining has helped to preserve the _Origin_’s accessibility, but its relative paucity of examples was particularly uncharacteristic of Darwin. _Variation,_ with its accumulation of evidence about everything from the webbing between dogs’ toes to the weight of gooseberries, was much more typical; in addition, it placed Darwin firmly—indeed, irretrievably—within his time, rather than in an achronological limbo reserved for intellectual heroes. As a graduate student from the People’s Republic of China told me several years ago, after having participated in a seminar that read excerpts from _Variation_ and _The Expression of the Emotions,_ if the leaders of his government knew that Darwin had written such books, he would not be officially admired.