The Gene: An Intimate History - Siddhartha Mukherjee Page 0,19

seems to me if he had given it a casual thought, he surely would never have dreamt of “pangenesis.”

—Alexander Wilford Hall, 1880

It is a testament to Darwin’s scientific audacity that he was not particularly bothered by the prospect of human descent from apelike ancestors. It is also a testament to his scientific integrity that what did bother him, with far fiercer urgency, was the integrity of the internal logic of his own theory. One particularly “wide blank” had to be filled: heredity.

A theory of heredity, Darwin realized, was not peripheral to a theory of evolution; it was of pivotal importance. For a variant of gross-beaked finch to appear on a Galápagos island by natural selection, two seemingly contradictory facts had to be simultaneously true. First, a short-beaked “normal” finch must be able to occasionally produce a gross-beaked variant—a monster or freak (Darwin called these sports—an evocative word, suggesting the infinite caprice of the natural world. The crucial driver of evolution, Darwin understood, was not nature’s sense of purpose, but her sense of humor). And second, once born, that gross-beaked finch must be able to transmit the same trait to its offspring, thereby fixing the variation for generations to come. If either factor failed—if reproduction failed to produce variants or if heredity failed to transmit the variations—then nature would be mired in a ditch, the cogwheels of evolution jammed. For Darwin’s theory to work, heredity had to possess constancy and inconstancy, stability and mutation.

Darwin wondered incessantly about a mechanism of heredity that could achieve these counterbalanced properties. In Darwin’s time, the most commonly accepted mechanism of heredity was a theory advanced by the eighteenth-century French biologist Jean-Baptiste Lamarck. In Lamarck’s view, hereditary traits were passed from parents to offspring in the same manner that a message, or story, might be passed—i.e., by instruction. Lamarck believed that animals adapted to their environments by strengthening or weakening certain traits—“with a power proportional to the length of time it has been so used.” A finch forced to feed on hard seeds adapted by “strengthening” its beak. Over time, the finch’s beak would harden and become pincer shaped. This adapted feature would then be transmitted to the finch’s offspring by instruction, and their beaks would harden as well, having been pre-adapted to the harder seeds by their parents. By similar logic, antelopes that foraged on tall trees found that they had to extend their necks to reach the high foliage. By “use and disuse,” as Lamarck put it, their necks would stretch and lengthen, and these antelopes would produce long-necked offspring—thereby giving rise to giraffes (note the similarities between Lamarck’s theory—of the body giving “instructions” to sperm—and Pythagoras’s conception of human heredity, with sperm collecting messages from all organs).

The immediate appeal of Lamarck’s idea was that it offered a reassuring story of progress: all animals were progressively adapting to their environments, and thus progressively slouching along an evolutionary ladder toward perfection. Evolution and adaptation were bundled together into one continuous mechanism: adaptation was evolution. The scheme was not just intuitive, it was also conveniently divine—or close enough for a biologist’s work. Although initially created by God, animals still had a chance to perfect their forms in the changing natural world. The Divine Chain of Being still stood. If anything, it stood even more upright: at the end of the long chain of adaptive evolution was the well-adjusted, best-erected, most perfected mammal of them all: humans.

Darwin had obviously split ways with Lamarck’s evolutionary ideas. Giraffes hadn’t arisen from straining antelopes needing skeletal neck-braces. They had emerged—loosely speaking—because an ancestral antelope had produced a long-necked variant that had been progressively selected by a natural force, such as a famine. But Darwin kept returning to the mechanism of heredity: What had made the long-necked antelope emerge in the first place?

Darwin tried to envision a theory of heredity that would be compatible with evolution. But here his crucial intellectual shortcoming came to the fore: he was not a particularly gifted experimentalist. Mendel, as we shall see, was an instinctual gardener—a breeder of plants, a counter of seeds, an isolator of traits; Darwin was a garden digger—a classifier of plants, an organizer of specimens, a taxonomist. Mendel’s gift was experimentation—the manipulation of organisms, cross-fertilization of carefully selected sub-breeds, the testing of hypotheses. Darwin’s gift was natural history—the reconstruction of history by observing nature. Mendel, the monk, was an isolator; Darwin, the parson, a synthesizer.

But observing nature, it turned out, was very different from experimenting with nature. Nothing about the natural