The Correspondence of Charles Darwin Volume I: 18211836
Past Masters: Mendel
Past Masters: Lamarck
Neuronal Man: The Biology of Mind
The catalog of Harvard’s Widener Library lists 184 books about Charles Darwin, his life and work (not counting 172 volumes of self-produced letters, autobiography, and scientific opera). On the subject of Gregor Mendel, there are only seventeen. The same disproportion is reflected in the books I have before me. Darwin is represented by a 702-page collection of letters all written before the age of twenty-seven, and a 449-page biography and subsequent history of the idea of evolution written by a professional biographer with no special expertise in the subject. When I contemplate yet another book about Darwin and Darwinism, I feel a bond of sympathy with the philistine Duke of Gloucester, whose reaction to a second volume of The Decline and Fall was, “Another damned, thick, square book! Always scribble, scribble, scribble, eh, Mr. Gibbon?” For Mendel on the other hand, the services of Vitezslav Orel, a great authority who has spent more than twenty-five years in historical research on the subject, have been obtained to produce a mere one hundred pages as part of a series of lives of the intellectual saints running from Aquinas to Wyclif.
As a population geneticist professionally concerned with Mendel’s mechanism for the inheritance of variation and with Darwin’s theory of evolution by selection of that variation, I have long found the vast disproportion in interest between the two to be paradoxical. While several explanations come to mind, none seems sufficient.
First, it might be argued that Darwin’s popularity on the intellectual market is a classic case of consumer sovereignty. People are greatly concerned with the place of human beings in the universe, so the materialist theory of evolution continues to agitate and fascinate all concerned. After all, the first printing of On the Origin of Species was immediately sold out, and interest has hardly died out since, as evidenced by the legal and journalistic trials still in progress in America. But the preoccupation of the literate middle classes and the fundamentalist masses with human uniqueness cannot explain the behavior of biologists, historians, and philosophers. While the hundredth anniversary, in 1959, of the publication of Origin of Species and the centenary, three years ago, of Darwin’s death were the occasions for large numbers of international symposiums and their attendant publications, the 1965 centennial of Mendel’s paper was lightly commemorated except in Czechoslovakia, and the centenary last year of his death went completely unnoticed by the institutions of science. The Journal of the History of Biology would have to close its editorial offices if it were not constantly supplied with more and yet more about Darwin, but the Folia Mendeliana, almost a one-man industry of Dr. Orel’s, appears only annually and is hard to find. In recent years, philosophers of science have abandoned physics for the richer and more complex domain of biology which, God knows, needs their help, but they have almost all taken Darwinism as their focus of interest. The deep epistemological problems in heredity and development have been left largely to the philosophical naïfs who practice the science.
Second, it might be claimed that Mendel’s discovery was intrinsically less interesting, especially from a philosophical point of view, than Darwin’s. The uncovering of the actual mechanism of heredity might be terribly important, but it is only a question of the mechanics, of particular gears and levers. But precisely the same can be said of Darwin. Although Origin of Species made evolution popular, Darwin certainly did not invent the idea. Indeed, a good case is made by L.J. Jordanova that, if any biologist should be considered the father of evolutionary theory, it is Lamarck. Most French intellectuals have regarded the Anglo-Saxon infatuation with Darwin as a typical piece of chauvinism. Darwinism is, if anything, a particular mechanism for evolution. That mechanism is the differential rate of reproduction, under pressure from the environment, of different sorts of individuals within a population. Moreover, the success of Darwin’s mechanical explanation of evolution depends critically on Mendel. Had heredity turned out to have a fundamentally different basis, Darwin’s idea, ingenious though it was, would have been wrong.
The problem is that natural selection among variant types causes the population to lose variation as the superior type comes to characterize the species. That is, selection destroys the very population variation that is the basis for its operation. Evolution would then soon come to a stop if there were not some continued source of variation among individual organisms. If heredity takes place by a blending mechanism, either by the mixing of blood or other fluids, then any new variation that arises would be immediately diluted out by the process of mating and the production of intermediate hybrids. Darwin was acutely conscious of this problem of the loss of variation from blending inheritance and the constant need for new sources of variants. In later editions of the Origin, he allowed for the possibility that heritable variation could be directly induced by environmental action. That is, he took in Lamarck’s view that acquired traits could be inherited, which is fatal to the whole Darwinian project of explaining evolution by a variational rather than a transformational mechanism. Mendelism saved the day.
The central core of Mendelism is the distinction between the appearance of an organism (the phenotype, in modern jargon), which may indeed be a blend of the characteristics of its parents, and the physical state of the factors inherited from each parent (the genotype), which remain physically discrete and unmixed. Just as Seurat’s Grande-Jatte gives the appearance of blended pigments from a close juxtaposition of small dots of pure color which are then visually fused by the physiology of the observer, so the physiology of development fuses, at the level of the whole organism, the pointillism of heredity.
Mendel’s realization of this distinction came from his experimental crosses with garden peas. When he crossed two truebreeding varieties that differed markedly in some characteristic, say, flower color, the offspring were uniform in appearance, which is precisely what one would expect from a mixture of two varieties. In Mendel’s case, there was the minor complication that the offspring all resembled one of the two parents rather than being intermediate between them, but this is the exception rather than the rule in most organisms. Thus, when Mendel crossed red-flowered and white-flowered garden peas, the offspring were all red-flowered. Had he worked with the sweet pea, Lathyrus odoratus, rather than the edible pea, Pisum sativum, the offspring would all have been pink. Whatever the color of the offspring flowers, the uniformity among individuals is precisely what one would predict from simple notions of the mixing of heredity. One would also predict that, if the uniform hybrids were crossed with each other, they would once again produce uniform offspring, and so on, without end.
But that is not what happened. When Mendel crossed these uniform hybrids with each other, he recovered in the next generation some plants with white flowers, like one of the two grandparents. From the reappearance of grandparental characteristics, apparently uncontaminated by their passage through the hybrids, and from the exact and repeatable ratios of types appearing among the offspring, Mendel constructed the two principles of heredity—principles that Darwinism needed to make it a workable theory. First, the factors that are passed from parent to offspring in heredity—what we now call genes—are particulate and maintain their individuality despite their interaction with other genes in the development of an organism. That is, the physical basis of heredity is discrete, like the elementary quantum of physics, rather than continuous.
Second, in the process of the formation of sperm and eggs in a hybrid organism, the genes that have been mixed together in that hybrid detach from each other and are parceled out to separate sperm and egg cells. That is the principle of segregation. Those two principles guarantee that if different variants in a population mate, even though their immediate offspring may be uniform and intermediate between the parents, in later generations the variation will reappear as a consequence of segregation. Thus new variation will not be submerged and diluted by the process of mating but will always be available for selection. Mendel’s principle of segregation is the rock on which the theory of evolution by natural selection is built.
The real epistemological revolution wrought by Darwin was, in fact, identical with that created by Mendel. That identity can best be seen in the contrast with Lamarck, who was concerned with the problems both of evolution and of heredity. Jean-Baptiste Lamarck was in many ways characteristic of the intellectual movement of the French Revolution. He was a deist, he accepted La Mettrie’s homme machine, rejecting the soul, and assumed that material principles underlay all natural and human phenomena. He combined with his materialism, however, the eighteenth-century commitment to natural philosophy, to the principle that all of nature reflects a few general organizing principles. Jordanova discusses these principles and illustrates how they bore on the biological problems of evolution, taxonomy, and heredity, but, in her treatment, their origin remains mysterious. They seem a priori, or at least broad generalizations from a small base of observations. Some of the organizing principles that Lamarck espoused, such as the effect of use and disuse of organs, certainly hold across a reasonable domain of phenomena. Muscles do atrophy if they are not exercised and bones do grow larger and thicker at points where muscles are attached and produce tension. But brains do not atrophy with disuse and an important current theory of neurobiology (see below) actually maintains the opposite.
Other of Lamarck’s principles, like the inheritance of acquired characteristics, were simply a priori or based on unexamined tradition. So giraffes’ necks may indeed grow a bit longer if they stretch them to reach the tops of trees, but that change is not passed on to future generations. It took Darwin to see that giraffes that happened to be born with long necks were better able to survive than those without them. What Lamarck had in common with all natural philosophy was a typological view of phenomena, what Ernst Mayr has called “essentialism.” In general, this meant that the ontological sources of similarity between things were seen as different from the ontological sources of differences. In particular, all members of a species were held to share unalterable properties that were intrinsic to the organisms, while differences between individual members were accidental consequences of environmental modification and were subordinate to the constant features. The problem of understanding the similarities was seen as fundamentally separate from the problem of the origin of superficial differences. It was the abolition of this distinction between the ontological sources of similarity and of difference that marked the epistemological break of Darwin and Mendel.
As I have argued in some detail in a previous review in these pages, 1 Darwin changed the object of study in evolution from the type of a species to the actual variation among individual organisms within the species. The motive power for a change in the average properties of the species was in the differences from the average displayed by the organisms themselves. Thus typical differences between species in space and in time arise by the accumulation of differences that were already present as variation within a species at any one place and time. But precisely the same contrast of similarities and differences permeated the study of heredity. Before Mendel, all studies of inheritance took heredity, that is, the passage of similarity between parents and offspring, to be different from, and antithetical to, the phenomenon of variation between individuals. The object of study was not the individual organism and its variant properties but the average or collective description of groups of progeny. For the predecessors of Mendel, the appearance of white-flowered plants among the progeny from the cross of the red-flowered plants implied a source of differences between organisms that was itself different from and obscured the action of the forces of heredity. For some, it was evidence of environmental malfunction. For others, it was the consequence of a poorly specified “force of atavism.” It remained for Mendel to use the very occurrence of different types among the offspring of a single cross as the key to the laws of inheritance.
"Darwin's Revolution," The New York Review (June 16, 1983).↩
“Darwin’s Revolution,” The New York Review (June 16, 1983).↩