Darwinism | Encyclopedia.com

The term Darwinism has both a narrow and a broad meaning. In the narrow sense, it refers to a theory of organic evolution presented by Charles Darwin (18091882) and by other scientists who developed various aspects of his views; in the broad sense, it refers to a complex of scientific, social, theological, and philosophical thought that was historically stimulated and supported by Darwin's theory of evolution. Biological Darwinismthe first sensewas the outstanding scientific achievement of the nineteenth century and is now the foundation of large regions of biological theory. Darwinism in the second sense was the major philosophical problem of the later nineteenth century. Today, Darwinism no longer provides the focus of philosophical investigation, largely because so much of it forms an unquestioned background to contemporary thought.

Darwin's theory is an example of scientific innovation that has had reverberations into the farthest reaches of human thought. It is fair to say that every philosophical problem appears in a new light after the Darwinian revolution. In order to outline the connections between biological and philosophical Darwinism, it will first be necessary to describe Darwin's own views and to discuss various criticisms that were directed against them. It will then be possible to describe Darwinism in the broader sense, and to distinguish the various ways in which the scientific theory has afforded material for philosophical inquiry.

The theory of the origin of species by means of natural selection was the discovery of Darwin and Alfred Russel Wallace (18231913). Both Darwin and Wallace had stated the theory in a series of papers delivered before the Linnaean Society on July 1, 1858. The members of the Linnaean Society listened without enthusiasm and apparently without much understanding, but in fairness to them, it should be observed that Wallace and Darwin did not present their theory forcefully on this occasion. Some of the shattering implications of the theory were not drawn in detail, and the evidence in its support, which Darwin in particular had amassed, was barely hinted at. Wallace's paper "On the Tendency of Varieties to Depart Indefinitely from the Original Type" was a discussion of a widely accepted argument in favor of the "original and permanent distinctness of species," namely, that the varieties that are produced by artificial selection in domesticated species never vary beyond the limits of the original wild species, and that whenever artificial selection is relaxed, the domesticated varieties revert to the ancestral form. These facts were interpreted by naturalists as evidence for an innate conservative tendency in nature that kept all variation within the bounds defined by the unbridgeable gaps between species.

But, Wallace argued, the view that artificial selection can produce only new varieties, never new species, rests on the false assumption that naturalists possess a criterion for distinguishing the species from the variety. Moreover he stated, "This argument rests entirely on the assumption that varieties occurring in a state of nature are in all respects analogous to those of domestic animals, and are governed by the same laws as regards their permanence or further variation. But it is the object of the present paper to show that this assumption is altogether false." Overproduction, together with heritable variations, some of which are better adapted to the circumstances of life, will tend to make varieties depart indefinitely from the ancestral type, bringing about changes that will eventually amount to the origin of a new species. Wallace accounted for the reversion of domestic varieties by pointing out that the ancestral type is better adapted to life "in a state of nature," and consequently the very same principles that bring about progress in nature also bring about the reversion of domestic varieties.

Wallace aimed his argument precisely at the philosophical presupposition that for so long had stood in the way of a proper interpretation of natural selection, namely, that the speciesbeing the exemplar of a divine archetypeis as well adapted as it could be and, consequently, that variation away from the type will automatically be selected against. Natural selection, according to this interpretation, is an agency of permanence, not change. One of Wallace's, as well as Darwin's, most original contributions consisted in breaking the hold of this idea.

Wallace's argument is implicit in Darwin's Linnaean Society papers, but the focus is different. Instead of challenging accepted opinion, Darwin added up well-known facts. With great eloquence he described the prevalent overproduction of animals and plants: "Nature may be compared to a surface on which rest ten thousand sharp wedges touching each other and driven inwards by incessant blows." The wedges are held back by large numbers of "checks" that bring about the death, or prevent the mating, of individuals. "Lighten any check in the least degree, and the geometrical powers of increase in every organism will almost instantly increase the average number of the favored species." He called attention to the extreme heritable variability of animals under domestication. In nature there is also variation, although no doubt not as much. Some variants will be better adapted to their environments than others and will tend to survive and propagate. "Let this work of selection on the one hand, and death on the other, go on for a thousand generations, who will pretend to affirm that it would produce no effect?" To the effects of this natural selection, Darwin added the effect of "the struggle of males for females."

Both Wallace and Darwin had stated the essence of the theory of the Origin of Species (1859). The Origin itself is mainly a sober, scrupulously fair, and thoroughly documented elaboration and defense of the doctrine of natural selection presented in the Linnaean Society papers. Darwin set out to accomplish three things: (a ) to show that evolution has in fact occurred; (b ) to describe the mechanism of evolution; and (c ) to account for the major facts of morphology, embryology, biogeography, paleontology, and taxonomy on the evolutionary hypothesis.

Darwin freely admitted that we do not directly observe the process of evolution. The time needed even for the origin in nature of a new variety is far too long. Consequently, the case for the occurrence of evolution is simply the same as the case for its scope and mechanism, and Darwin did not have access to direct evidence for the efficacy of natural selectiona gap that was not filled until the twentieth century. Darwin argued that life is too short for direct evidence but that certain facts force the conclusion upon us that there must be evolution; and if we adopt the hypothesis, a wide range of hitherto unconnected facts may be given a uniform explanation.

Darwin described three mechanisms that tend to effect the evolution of populations. These are natural selection, sexual selection, and the inheritance of characteristics acquired during the lifetime of the individual organism.

In the Origin Darwin placed the greatest weight on evolution by natural selection. It operates in conjunction with sexual selection and the inheritance of acquired characters and, Darwin argued, there are some features of organisms that could have developed only by natural selection. Indeed, it seems that the theory of natural selection was partially inspired by his obser-vations on the Beagle voyage (18311836) of local variations, particularly in the islands of the Galpagos Archipelago, that could not be accounted for on Lamarckian grounds.

The theory of natural selection as Darwin presented it may be summarized as follows: (1) Populations of animals and plants exhibit variations. (2) Some variations provide the organism with an advantage over the rest of the population in the struggle for life. (3) Favorable variants will transmit their advantageous characters to their progeny. (4) Since populations tend to produce more progeny than the environment will support, the proportion of favorable variants that survive and produce progeny will be larger than the proportion of unfavorable variants. (5) Thus, a population may undergo continuous evolutionary change that can result in the origin of new varieties, species, genera, or indeed new populations at any taxonomic level. Darwinian natural selection may accordingly be defined as a differential death rate between two variant subclasses of a population, the lesser death rate characterizing the better-adapted subclass.

Darwin was careful to present evidence for every hypothesis in his account of natural selection. It was especially necessary to argue that natural populations do exhibit the requisite amount of variation and that the variation is heritable. He cited, among other things, the extreme variability of domestic plants and animals and the well-known fact that new varieties can be propagated. He admitted that the causes of variation were unknown; but he argued that changing environmental conditions greatly increase variability by action on the reproductive system, thereby providing material for natural selection when it is most needed. This is "indefinite variability." In addition, there is "definite variability," due to the direct action of the environment on the body of the organism. "Definite variations" are heritable; they provide material for natural selection and, being responsive to the environment, are more likely than chance variations to be adaptive.

"The laws governing inheritance," he remarked, "are for the most part unknown." This lack of knowledge turned out to be the most serious obstacle to the further development of the theoretical foundations of selection theory in the nineteenth century; but, as Darwin noted, although the laws of inheritance were unknown, a number of the phenomena of inheritance were known, and those were probably all that were required for the theory of natural selection. Most important is the obvious fact that progeny bear an overwhelming resemblance to their parents, although they differ in some degree. In addition, Darwin was familiar with the intermittent appearance of hereditary characters, with sex-linked and sex-influenced characters, and with the tendency for a character to appear in the progeny at the same developmental stage that it appears in the parents.

For natural selection to be an agency of change rather than an agency of permanence, it is necessary that some variations from the ancestral type represent better adaptations. Darwin pointed out that, in fact, every organism could be better adapted to its ordinary environment; and that, moreover, environments change.

Pre-Darwinian taxonomy ascribed a very special significance to the species, as against varieties, genera, and the higher taxonomic groups. The species was regarded by the pious as the unalterable work of God; the limits laid down by the diagnostic features of any species established the limits of possible variation within the species. Thus, although any biologist would be willing to countenance the origin of new varieties or subspecies, brought about by the operation of biological laws, most were unwilling to admit the possibility of the origin of new species by natural processes. The title of Darwin's book was aimed precisely at this conception. Like Wallace, he argued that there is no difference in principle between the diagnostic characters of varieties and species; therefore, to admit the origin of new varieties amounts to admitting the possibility of new speciesand if new species appear, so may new genera, families, and so on. He cited the existence of "doubtful species"groups that cannot be definitely placed at either the variety or species leveland the general inconsistency of taxonomists in the identification of species.

In the Linnaean Society papers Darwin described the second mechanism of evolution as the "struggle of males for females." The theory was developed further in the Origin, and it occupied some two-thirds of the pages of his Descent of Man and Selection in Relation to Sex (1871). In these later statements of the theory, the struggle of males for females is a special case of a more general phenomenon. Suppose that a population is divided in some proportion between males and females and suppose for the sake of simplification that all of the males and females are equally well endowed for the struggle for survival. Now, Darwin argued, it may happen that either the males or females are unequally endowed with some characteristic that will increase their propensity to leave progeny. There will then be selection in favor of that characteristic, even though it will not be favored by natural selection. All such cases Darwin calls "sexual selection." It is clear that different sorts of characteristics can influence the probability of having offspring. Some individuals, for example, may possess behavior patterns that lead to the fertilization of a larger percentage of eggs or have more efficient organs of copulation. Or they may have some advantage in the competition for matesmigratory male birds may arrive early at the breeding grounds and be ready to receive the more vigorous females, leaving the culls for their tardy brothers; or the females may for some reason prefer plumage or displays of a certain character; or some males may aggressively drive away other males; and so on. Finally, some characteristics that are also useful in the struggle for survival might also be useful in the competition for mates; for example, the antlers of male deer may do double duty against both rivals and predators.

Darwin appeals to sexual selection in order to account for the evolution of such things as mating rituals and secondary sexual characteristics, such as breeding plumage in birds. He regards it as especially significant in the evolution of man. The loss of body hair, for example, is attributed to systematic choice among man's ancestors of mates that exhibited large regions of bare skin.

Darwin's work was plagued by ignorance and misinformation concerning the laws of heredity. The principles of segregation and independent assortment, which form a cornerstone of contemporary evolution theories, were discovered by Gregor Mendel in 1864; but his paper remained unnoticed until 1900. Moreover, although "sports" were well known to biologists, the concept of mutation had not been clearly formulated. Consequently, the modern theory of the origin of genetic variation in populations was not available to Darwin; instead, he suggested that some variations are due to the action of the environment on the germplasm and that others are due to the effects of use and disuse. For example, if an animal's skin is tanned by sunlight, this may induce changes in its germplasm that will result in its progeny possessing pretanned skin; or if a wolf develops his muscles by chasing rabbits, his pups may inherit larger muscles. These mechanisms, if they exist, would account for some variability. But they would also account for some evolutionary change even in the absence of natural or sexual selection. Since, accordingly, there seemed to be no sound reason for rejecting the inheritance of acquired characters and since the doctrine would aid in explaining both variability and evolutionary change, Darwin was led to adopt it and to give it increasing weight in his later years. This aspect of Darwin's views is often labeled Lamarckism, but the Chevalier de Lamarck himself, although he did accept the inheritance of the effects of use and disuse, did not accept the doctrine of the direct action of environmental factors on the germplasm.

It is clear that Darwin regarded his theory as revolutionary. He believed that all the traditional branches of biology would be transformed and deepened; familiar phenomena would take on a new significance; apparently unconnected facts could be regarded as mutually related. Even the vocabulary of the older biology would acquire new meanings: "The terms used by naturalists, of affinity, relationship, community of type, paternity, morphology, adoptive characters, rudimentary and aborted organs, etc., will cease to be metaphorical, and will have a plain signification." Natural history would acquire the fascination, not of a catalog of curiosae, but of a labyrinth that may be charted.

When we no longer look at an organic being as a savage looks at a ship, as something wholly beyond his comprehension; when we regard every production of nature as one which has had a long history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, when we thus view each organic being, how far more interestingI speak from experiencedoes the study of natural history become!

And not only would the old biology be put on a new foundation; whole new fields of research would become possible. For example, "Psychology will be securely based on the foundation of the necessary acquirement of each mental power and capacity by gradation. Much light will be thrown on the origin of man and his history."

The major part of the Origin is devoted to the detailed application of the theory of natural selection to a range of biological phenomena. It is impossible to give more than a general impression of the thoroughness, detail, and diversity of Darwin's evidence. The modern reader cannot fail to be impressed not only by Darwin's immense learning but also by his subtlety of insighthis ability to locate those phenomena that lend his theory the most striking support.

The Origin as a whole provides, on the one hand, a sweeping portrait of the history and biology of living things, a portrait whose internal balance and consistency are easily discernible. On the other hand, Darwin fills selected regions of his portrait with careful detail, exhibiting the applicability of his theory to a variety of phenomena. These two aspects of his work constitute both the argument for the fact of evolution and the argument for the truth of his account of its mechanisms.

In the broad portrait Darwin shows how the main facts of known fossil successions, the relation of living fauna and flora to recent fossil forms, the geographical distribution of species, the connection between morphology and function, and the major features of embryological development are explicable by his theory. He applies it in detail to such phenomenato mention only a fewas rudimentary organs, insect metamorphosis, the divergence of island and mainland forms, and sexual dimorphism. He provides us with a discussion of taxonomy that is philosophically superior to many contemporary accounts, arguing, among other things, in favor of the special significance for the taxonomist of embryological and phylogenetic studies.

Darwin was always sensitive to the effect that his views might have on the general public. In composing the Origin he decided to avoid the whole topic of man's evolution; the book would be a sufficiently bitter pill without explicitly treating a subject that was "so surrounded with prejudices." His only explicit reference to man was the remark quoted above, that "light will be thrown on the origin of man and his history." Darwin's successors, however, were not so cautious. Sir Charles Lyell (17971875) discussed the question in 1863. Shortly thereafter, Wallace published his paper "The Origin of Human Races and the Antiquity of Man Deduced from the Theory of Natural Selection." T. H. Huxley (18251895) and a number of Continental morphologists, particularly Ernst Haeckel (18341919), produced a series of studies aimed at showing the similarity of man and the anthropoid apes and giving speculative reconstructions of man's ancestry. Thus, by the date of Darwin's Descent of Man (1871), the controversy over man was in full swing, and there were already a number of alternative theories that Darwin had to consider, such as whether the races of men are distinct species.

Darwin showed a wise unwillingness to acknowledge any known nonhuman species, living or extinct, as ancestral to man. We have so far examined, he argued, only animals that have diverged from the prehuman stock. For instance, the anthropoid apes and man have a common ancestor, but its remains have not been found. Nor did he identify species that are ancestral to the primates, the mammals, or even the vertebrates. He did trace a general line of descent: Old World ape, a lemurlike animal, some "forms standing very low in the mammalian series," marsupials, and monotremes. No true reptile is an ancestor of man. All the classes of vertebrates may have been derived from a remote ancestor similar to the larvae of the tunicates. With a flash of romanticism, Darwin wrote: "In the lunar or weekly recurrent periods of some of our functions we apparently still retain traces of our primordial birthplace, a shore washed by the tides."

In the Descent of Man evolution by the inheritance of acquired characters and by sexual selection plays a larger role than in the Origin. Darwin admitted that he had been accused of overrating the importance of natural selection, but added, "whether with justice the future will decide." His relative retreat from natural selection was probably occasioned by two factors: first, his doubts as to whether Earth is old enough for evolution by natural selection without substantial help from faster mechanisms; second, his belief that man is in many ways less the child of violent nature than his ancestors, a belief that requires considerable appeal to sexual selection and to the development of moral and spiritual qualities through social usage.

In spite of the resistance that Darwin's theory aroused on other than scientific grounds, the weight of his arguments was largelybut with many notable exceptionssufficient for the younger generation of biologists. In 1872, in the sixth edition of the Origin, Darwin was in a position to write, "At the present day almost all naturalists admit evolution under some form." It was, like any novel and important theory, carefully scrutinized for empirical weaknesses. We shall describe the major ones and indicate how they were dealt with.

The most damaging scientific objections were the following:

The first two objections were commonly raised in the nineteenth century; they are genuine questions that require some sort of answer. Darwin, however, was not in a position to answer them in a way that would satisfy everybody, since the weight that one assigns to them depends in part upon personal preference.

With regard to the first objection Darwin pointed out that natural selection cannot be directly observed; we can only present indirect evidence in its favor. On this point he was mistaken. Natural selection has been directly studied in the twentieth century, both experimentally (in fruit fly populations, for example) and in nature (for instance, the development of so-called industrial melanism). But even today Darwin's and Wallace's contention that evolution by natural selection can pass the species limit has no direct support. Darwin recognized, however, that it is no fatal objection to a theory if some of its components are not subject to direct verification.

On the second criticismthe absence of forms intermediate between speciesDarwin had a double-barreled answer. He admitted that, for instance, we know of no forms intermediate between man and the apes. But we have innumerable examples of species that are in process of giving rise to new species, namely, those that have varieties or subspecies. These polytypic species (as they are now called) are intermediate between other species which, to be sure, have not yet evolved, but which are in process of evolving.

When it was further objected that we ought to have better examples of demonstrable ancestors of existing species, Darwin appealed to the incompleteness of the fossil record. This is the correct answer, but one that is hardly satisfying to a skeptic. Again, the weight that one would assign to the objection depends upon personal preference.

Darwin was well aware of the difficulty in accounting for the origin of structures that would be useless, even deleterious, until they were essentially complete. The eye, he wrote, gave him "a cold shudder." In such cases as the eye, however, he had no alternative but to appeal to natural selection. Therefore, he was compelled to argue that in point of fact all the earlier stages in the evolution of the eye were useful in the struggle for survival. Darwin himself provided us with the standard textbook example: he constructed a plausible sequence of stages that could have led to the human eye. Each stage is a functional eye; and something similar to each stage does exist in one or another living species. The criticism has the form, "Such and such could not have happened." It can be countered piecemeal, by showing in a variety of cases how it could have happened.

A great many of Darwin's critics accepted the fact of evolution but entered reservations concerning his account of the mechanisms of the process. The reservations were of several types. Some rejected "Lamarckism," by which they meant simply the inheritance of acquired characters; they were known as the Neo-Darwinians. Others doubted that there was such a process as sexual selection. Still others, however, believed that there must be an evolutionary process that Darwin had not identified at all. The evidence consisted in the existence of apparently nonfunctional evolutionary trends. Trends that continue over long periods and that are relatively straight-linedfor example, increasing size in horses and increasing length of sabers in the saber-toothed catcame to be called orthogenetic trends. The question was whether orthogenetic trends could be accounted for on Darwinian principles.

Wallace argued (in "Geological Climates and the Origin of Species," Quarterly Review, 1869) that the development of man's brain could not be so accounted for. Man's apelike ancestors, he argued, had reached a certain stage of evolution and then, over a period of some ten million years, remained largely unchanged except for a steady orthogenetic increase in the size and complexity of the brain. This was an unprecedented episode in the history of life, for it freed man from those ordinary pressures of natural selection that so often led to close specialization and ultimate extinction. Moreover, the brain acquired abilities that could not have been exercised in a primitive environment, such as the power to construct speculative systems of ideas or the insight into spiritual reality. These are present in modern man, but would have been useless in man's primitive ancestors. Natural selection operates only on abilities that are actually so exercised as to give an advantage in the struggle for life. "An instrument," Wallace concluded about the brain, "has been developed in advance of the needs of its possessor." Later he wrote: "A superior intelligence has guided the development of man in a definite direction, and for a special purpose, just as man guides the development of many animal and vegetable forms." Thus we avoid the "hopeless and soul-deadening belief" that man is the product of "blind eternal forces of the universe."

Darwin looked upon this as a failure of nerve, a hankering after miraculous origins for man. "I can see no necessity for calling in an additional and proximate cause in regard to man," he wrote in a letter to Wallace. Nevertheless, Wallace's position, fitting as it did the efforts of many theologians to come to grips with Darwinism, gained a number of adherents, and although the main line of evolutionary theory has bypassed it, even now versions of Wallace's position turn up from time to time.

Wallace had argued that the evolution of the brain was an orthogenetic trend that outstripped its usefulness. Others argued that trends sometimes continued even after they had become positively deleterious. A favorite example was the teeth of the saber-toothed cat, which, it was alleged, were valuable as weapons up to a certain length, but which finally became detrimental by interfering with feeding. There would be selection against increased tooth length under these conditions; consequently, it was argued, some cause other than natural selection must have operated. A variety of theories were proposedfor example, those of Karl Ngeli (18171891) and E. D. Cope (18401897). These theories posited an otherwise unknown internal principle of change, which was compared to the laws of embryological development, to the principle of inertia, or, as with Henri Bergson, to creative spiritual activity. Since the theories accounted for nothing other than the alleged orthogenetic trends, they have always had a peripheral position in the history of evolutionary thought. Moreover, subsequent analysis of orthogenesis has shown that in most cases the trends are in fact adaptive; and in those cases where they are not adaptive, contemporary theory provides various possible sorts of explanation compatible with the doctrine of natural selection, such as the explanation that if a trend affects only adults past the breeding age, it will not be selected against.

In 1865 William Thomson, Lord Kelvin, published a paper titled "The Doctrine of Uniformity in Geology Briefly Refuted." Its argument was aimed at Lyell and his followers, who had maintained that Earth as we now find it is not the result of a series of catastrophes, but is the outcome of the ages-long operation of geological processes that we can still observe. This viewpoint, known as uniformitarianism, was widely accepted among geologists even before the publication of the Origin, having been impressively established in Lyell's Principles of Geology (1834). It was in fact an earlier application of the idea of evolution. But uniformitarianism required vast reaches of time; consequently, Kelvin was prodding its weakest point when he argued that Earth could not be as old as the geologists supposed. Grant, Kelvin argued, that Earth was once a molten sphere; then it could not have solidified much over twenty million years ago, or it would now be cooler, through dissipation of its heat, than we actually find it. The biological consequences were clear: there was not enough time for evolution to have produced the forms we now see.

Darwin was deeply concerned by this reasoning. As far as he could tell, it was perfectly sound; on the other hand, he was perfectly convinced that Earth had supported life for a much longer time. His later emphasis on Lamarckism was probably an attempt to provide an evolutionary process that was swifter than natural selection. But this was a half measure; in fact, Darwin simply swallowed what he believed to be a contradictiona not uncommon occurrence in the history of science. It turned out that Kelvin's argument was mistaken, since he was unaware of an additional source of heat within Earth, namely radioactive decay.

As noted above, the evolutionists of the nineteenth century worked in ignorance of the principles of genetics discovered by Mendel; this lack was by far the most serious theoretical gap in the Darwinians' arguments. It now appears that no fundamental innovation in evolutionary theory was possible until the gap was filled. Biologists of the nineteenth century accepted a rough theory of blending inheritance, that is, the view that the characteristics of the progeny of sexual crosses were intermediate between the characteristics of the parents. This theory was seldom explicitly defended, since everyone was familiar with a variety of phenomena that were incompatible with it, for example, blue-eyed children of brown-eyed parents. Nevertheless, when biologists theorized at all on the subject, the theory produced was ordinarily a vague and suitably guarded version of the theory of blending inheritance.

In 1867 Fleeming Jenkin ("The Origin of Species," North British Review ) pointed out that the blending theory was incompatible with the theory of natural selection as ordinarily presented by the Darwinians. He argued that if favorable variations appeared in a population, their characteristics, even if favored by natural selection, would soon be lost in the vast population pool by crossing with individuals of the normal type. Assume, for instance (as Jenkin did), that a white man is greatly superior to a black man and that a white man is shipwrecked on a black-populated island. "He would kill a great many blacks in the struggle for existence; he would have a great many wives and children. But can anyone believe that the whole island will gradually acquire a white, or even a yellow population?" Jenkin's argument in essence is this: the white man's children will be darker than their father; and it is impossible on the blending theory that their descendants could become lighter, whatever the effects of natural selection might be.

Again, Darwin was forced to admit the strength of a powerful objection that he was unable to counter directly. At best, he could only argue that natural selection would be effective if adaptive variations were sufficiently common; the black island could become white, for example, if there were a steady influx of shipwrecked sailors. He actually had no evidence that adaptive variations were sufficiently common; instead, he retreated more and more to the Lamarckian theory that variation is due to the effects of activity in the environment and would accordingly be largely adaptive.

Unlike the answer to Kelvin's objection, which could not have been offered in the nineteenth century, the answer to Jenkin was available but remained unknown except to a few, who did not see its significance. Mendel's paper on plant hybridization established an alternative to the blending theory of inheritance. Mendel showed that there were discrete genetic factors that pass unchanged from generation to generation and are hence not subject to Jenkin's swamping effect. Mendel had established that the character of these factors (genes) is not changed by other factors in the germplasm and that the factors segregate independently of one another in gamete formation. (He was unaware of the phenomenon of linkage.) Researchers of the literature on heredity recovered Mendel's work in 1900; and in 1904 William Bateson (18611926), in Genetics and Evolution, applied Mendel's laws to the theory of natural selection, thus answering Jenkin's objection.

The new genetics turned out to be far more significant for the theory of evolution than merely answering Jenkin's objection. The history of scientific Darwinism in the twentieth century was mainly the story of a series of advances in genetics, and the working out of their consequences for evolution. Mendel's laws were correlated with the behavior of the chromosomes in meiosis; the concepts of chromosome and gene mutation were introduced; linkage was discovered and understood; and statistical methods were employed in the analysis of the dynamics of genetic change in natural populations. One major gain of these developments was a systematic understanding of the origin and maintenance of genetic variabilitythe question that was so troublesome for Darwin. Another was the final decline of the Lamarckian aspect of Darwinism.

The Neo-Darwinians had already denied the inheritance of acquired characters, but their evidence against it, like the Neo-Lamarckians' evidence in its favor, was largely anecdotal. August Weismann (18341914) had presented the theory that life is essentially a continuous stream of germplasm that from time to time gives rise to whole organisms; the organisms die but the germplasm is immortal. The stream can divide (gamete formation) and merge (fertilization), thus accounting for variability. This view was employed by Weismann and others as a theoretical argument against the inheritance of acquired characters, for it is an easy step from the continuity of the germplasm to its independence of somatic influences. The emergence of Mendelism shed a new light on Weismann's theory. The mechanism of "immortality"self-replication of chromosomeswas elucidated, and evidence accumulated that the chromosomes were indeed uninfluenced, or influenced only randomly, by somatic factors.

We have considered Darwinism as a biological theory; we may now consider its wider intellectual connections. These are many and complex, so it will be necessary to select the most importantthose which now seem to be enduring ingredients of speculative thought or those which struck the people of the later nineteenth century with the greatest force. The differences between the climate of opinionthe ordinary presuppositions, ideas about the proper pattern of argument, assumptions as to proper method, in short, the worldviewof the mid-nineteenth and twenty-first centuries is large, comparable in degree to the differences between the Middle Ages and the Renaissance. Of course the change had many causes, but the advent and absorption of Darwinism, while in part an effect of other currents, was also one major cause.

We shall consider the connections of Darwin's theory in three major regions: scientific cosmology, theology, and social doctrine.

Scientists have general views about the way things are. The scientists of any historical period are likely to share a common set of views, with, of course, individuals differing over one or another point to some degree. These general views, insofar as they concern a subject matter of professional scientific interest and insofar as they are capable of influencing method, methodology, or empirical formulations, may be called cosmological. They differ from the ordinary statements of a science (for example, "organisms overproduce," "acquired characters are not inherited") in degree of determinateness. They are so formulated that they are exempt from immediate verification and falsification but subject to specification, by means of a series of semantical decisions, into determinate, verifiable propositions. A good example of such a cosmological proposition is "Nature makes no jumps," or "Nature has no gaps." Darwin, unlike many of his contemporaries, was fond of making this remark (in Latin); he employs it in the Linnaean Society papers and subsequently quotes it again and again. It constitutes part of Darwin's cosmology and is a point on which the nineteenth century was deeply divided. It is clear that the sense of the proposition is not sufficiently determinate, as it stands, for verification. But it can be construed to mean, for instance, that evolution is gradual or that the apparent gaps between living species can be filled if we consider a sufficient stretch of history.

These properties of cosmological belief have important implications. First, it is possible to arrive at a cosmology by a process akin to generalizationan empirical statement can be construed as the determinate form of an indeterminate proposition, which in turn can be applied to new subject matters. This is the formal pattern of the influence of science on cosmology. Second, the precise verbal formulation of a cosmological belief is relatively unimportant; indeed, it can affect thought without being explicitly formulated at all. For cosmological beliefs do not function as premises of empirical arguments; rather, they impart color to empirical argument, affecting its form and conceptual materials.

Darwin's biological theory was itself supported by prior developments in cosmological belief. The theory of evolution by natural selection did not occur to Darwin in an intellectual vacuum. Most important of these cosmological beliefs was uniformitarianism, the belief that nature operates everywhere and always by the same sorts of law. This view Darwin had imbibed from Lyell's Principles of Geology ; it became cosmological by construing the geological theory as exhibiting a general truth about the way things, including livings things, are. This particular belief is already a powerful stimulus to look at organic nature as the outcome of a historical process, although, to be sure, the belief does not entail this conclusion.

A second belief, which Darwin inherited and was seen to support, was the necessity of taking time seriously. This meant, among other things, that the past is long. By the date of the Origin there was little actual evidence on the age of Earth, let alone the age of the universe. Outside scientific circles, the prevailing view was that Earth and universe were the same age, something on the order of thousands of years. As long as this is accepted, evolution is evidently most improbable. Some geologists, in particular James Hutton (17261797), had, on the other hand, argued that Earth is infinitely oldan important argument, since it helped to accustom scientists to the possibility of vast stretches of time and change. Geologists after Hutton were willing to help themselves to as much time as they needed, and Darwin gladly followed suit.

Taking time seriously, however, gained a deeper meaning after the publication of the Origin, namely, that change is a fundamental feature of nature. This constituted part of the cosmology of every Darwinian. It meant that the process of change is not merely the reshuffling of preexisting materials in accordance with physical law but that the materials themselves are subject to alteration. For instance, as applied to biology it meant that the fundamental form, the species, did not merely exhibit eternal law but changed in such a way that new regularities of behavior replaced the old. In the favored terminology of the nineteenth century, we may say that taking time seriously meant that the laws of nature are subject to change.

Structures and patterns of behavior, then, have to be regarded as historically conditioned. This is the cosmological aspect of the most characteristic post-Darwin view of method, the insistence upon the investigation of origins, together with the view that such investigation can be scientific. Thus, we find the development of the idea of a human prehistory, the application of elaborate schemes concerning, as they were called, stages of developmentspiritual, social, political, moraland the belief that, at least in outline, the future of man may be successfully charted.

Pre-Darwinian biological theory was strongly influenced by the view that all living things are patterned after an eternal idea or archetype. This was held not only for the species but also for other taxonomic categories and for anatomical structures as well. Taxonomists were fond of describing, for example, the ideal vertebrate or mollusk; and morphologists described the ideal organ. One of the achievements of Darwinism was to break the hold of this notion on taxonomic and anatomical theory. Darwin was finally able to write, in Descent of Man, "A discussion of the beau ideal of the liver, lungs, kidneys, etc., as of the human face divine, sounds strange to our ears."

The expressed doctrines of theology are related to empirical propositions as cosmological doctrines are related to the natural sciences. The role of Darwin's theory as a generator of such indeterminate beliefs naturally is well exemplified in theology. On the one hand it was immediately taken to be in prima facie opposition to a number of theological doctrines, especially the following: the uniqueness of man as God's supreme creation; the importance of natural theology; and the dominant theory, in Protestant circles, that the Bible is an authoritative source of beliefs about the natural world.

The first theological reaction to Darwinism can only be described as one of outrage; but by the close of the century, theologians having decided that since they must live with Darwinism, they ought to love it, the outlines of a reconciliation had been sketched. Even further, Darwinism was allowed to guide the formation of a new brand of theology. We shall consider first the reaction.

As we have seen, Darwin's readers were quick to grasp the consequences of the Origin for man himself. These consequences immediately aroused the most intense feelings. These feelings were quite justified, for Christian theology demands that man be considered unique; and his uniqueness was universally interpreted as ontological separateness from the rest of creation. The geologist Adam Sedgwick (17851873), for example, spoke no more than common opinion when he wrote in 1850 that man is a barrier to "any supposition of zoological continuityand utterly unaccounted for by what we have any right to call the laws of nature." The Darwinians not only argued that man is continuous with the animal kingdom and subject to the laws of nature; they also asserted that his mental, moral, and spiritual qualities evolved by precisely the same processes that gave the eagle its claws and the tapeworm its hooks. Such opinions were a threat to the deepest level of Christian doctrine, and were bound to be, until man's uniqueness could be given a new theological interpretation.

Moreover, the furor over the animal nature of man was heightened, especially in Britain, by local circumstances. T. H. Huxley compared man and the ape with endless zest, knowing how the comparison annoyed his opponents. For apes and monkeys were thought to be oversexed and obscene; in addition, the British took very seriously the principle that a man's standing in the world is dependent on the standing of his ancestors. Thus the literature of the period is enlivened by comic remarks, such as, "Are you descended from an ape, Mr. Huxley, from your mother's or your father's side?" (Bishop Wilberforce) and "You can't wash the slugs out of a lettuce without disrespect to your ancestors" (John Ruskin). But the symbol of the ape squatting in one's family tree was no more than an expression of dismay at being swallowed up in the infinite forms of nature. The twentieth century did not fully regain its equanimity on this point. Pius XII wrote that a Catholic may accept a doctrine of evolution, but should beware of doubting that there was a first man and woman. And consider this passage from the speech of William Jennings Bryan at the Scopes trial (1925): "We are told just how many species there are, 518,900. and then we have mammals, 3,500, and there is a little circle and man is in the circle, find him, find man."

The edifice of traditional theology was touched at other points. Early-nineteenth-century theologians placed heavy weight on the cooperation of science and religion. The clergyman-naturalist was a familiar figure. It was thought that the intricacy and systematic interconnections of nature exhibited the handiwork of God; to study them was an act of piety. More specifically, natural teleology was the mainstay of natural theology. William Paley's Natural Theology (1802) is a good example. He holds that God's creation is totally good, that the organs of living things are almost perfect, that all animals have their just share of happiness, and that all this demonstrates with thousandfold certainty the existence and beneficence of God. An older natural theology tended to see evidences of God's design throughout nature; but Paley, and others after him, such as Thomas Chalmers in the Bridgewater Treatises (1834), rest their case on the structure of living things: consider, they suggest, the hand, the heart, the eye (especially the eye); they are complex and adapted for their functions to a degree that transcends all possibility of chance correlation.

By hindsight this attitude appears curiously self-defeating as well as vulnerable. The religiously inspired examination of organic adaptation was precisely one factor that led to Darwin's account of the origin of adaptation. His theory made the last citadel of divine teleology in nature untenable except, of course, for a few holdouts; but it was also widely interpreted as refuting all natural teleology, especially by the German materialists. "Chance" had been defined by Paley as "the operation of causes without design," and on this definition Darwinism leaves the origin of species to chance.

Theology in the middle half of the nineteenth century was especially vulnerable to Darwinism on a second point, namely, its extreme Biblicism and, even further, its literalism in biblical interpretation. It hardly needs saying that Darwinism is incompatible with any literal construction put upon either the Old Testament or the New Testament. The laity and most of the clergy, however, insisted upon such constructions. Matthew Arnold quotes the following as prevailing opinion in England: "Every verse of the Bible, every word of it, every syllable of it, every letter of it, is the direct utterance of the Most High"a view Coleridge describes as "Divine ventriloquism." The matter was not so extreme outside of Britain, but the fact remains that Protestant education and practice relied heavily on the study and interpretation of the Bible.

The intellectual compromise that gradually emerged seems obvious today; the problem was not to think of it but to accept it. It consists in admitting that man is part of nature and that he is indeed, even in his spiritual aspects, the outcome of an evolutionary process. But lowly origins do not detract from a unique present. And the process of evolution is either guided, as Wallace suggested, or is itself the mode and manner of God's creation. Indeed, it was sometimes argued that Darwinism provides us with an elevated conception of God. Canon Charles Kingsley, for example, wrote to Darwin as follows: "I have gradually learnt to see that it is just as noble a conception of the Deity to believe that he created primal forms capable of self-development , as to believe that He required a fresh act of intervention to supply the lacunas which He Himself had made." This passage is quoted by Darwin with some changes in later editions of the Origin. As Kingsley also put it, Darwin allows us to get "rid of an interfering Goda master-magician, as I call it" in favor of a "living, immanent, ever-working God."

The final step in this direction was to give God an even more intimate metaphysical connection with natural process. This step had been taken by previous philosophersBenedict (Baruch) de Spinoza and G. W. F. Hegel, for example; but it was repeated under the aegis of Darwinism by Bergson, Alfred North Whitehead, and a number of Protestant thinkers. The problem of a divine nature that is both perfect and yet incomplete is one contemporary heritage of Darwinism.

The social thought of the later nineteenth century drew so heavily from the theories of evolution that its major ideas became known as social Darwinism. The 1850s were a period of revolutionary fervor in the streets as well as the academies, and political ideologists seized on Darwin as their major intellectual spokesman. His views, or rather selected aspects of them, presented ideal material for application to ethical, economic, and political problems.

It is convenient to divide social Darwinism into a political right and left, using these terms in their rough, contemporary editorial-page sense. In adopting Darwinism to social questions, it must be admitted that the right wing had the best of the bargain. In Europe these were the men whose interests were vested in hereditary privilege and in the factories and institutions of the industrial revolution. On the grounds of these interests they defended themselves against any attempt to justify social revolution, governmental control, unionism, or socialism in any of its many nineteenth-century forms. The ideology that was developed, with the help of Darwinism, in order to facilitate this defense also committed them, in various combinations, against such things as child-labor legislation, poor laws, compulsory safety regulations, and public education. A similar ideology provided the United States with its justification for the undisturbed economic expansion, speculation, and competition that we associate with the robber barons.

On the other hand, Darwinism was employed by the social reformers. Karl Marx wanted to dedicate the first volume of Das Kapital to Darwin. George Bernard Shaw, although he criticized the theory of natural selection, defended his socialism with the help of his version of Bergson's creative evolutionism. The reformers saw Darwinism as the final demonstration that no particular economic or political institutionhowever hallowed by tradition or supported by existing theoriesneed be regarded as unalterable. The forms of society, like the forms of life, are local, temporary, and functional and may accordingly be changed (for the better) without shaking the foundations of the cosmos.

In short, the biology and cosmology of Darwinism was capable of being all things to all men. It enjoyed this status by virtue of its ability to inspire and lend a measure of apparent scientific support to the following major ideas:

(1) The vision of a science that was historical, and at the same time a rigorous application of natural law, inspired a new vision of a science of society. Herbert Spencer (18201903), whose evolutionism antedated the Origin, became the symbol of this ideal wedding of history and sociology. He drew elaborate comparisons between social structures and the forms of living organisms and saw societies as undergoing a progressive evolution in which egoism would be gradually replaced by altruism through a mechanism analogous to the inheritance of acquired characters. Sociology stood in relation to society as evolutionary biology stood to the phenomena of organic nature.

(2) The process of natural selection, interpreted as the survival of the fittest, provided a means for explaining social process. The American political economist William Graham Sumner (18401910), for example, saw society as the outcome of a social struggle in which each man, in pursuing his own good, can succeed only at the expense of others. The fittest in this social struggle are the ruthless, the imaginative, the industrious, the frugal. They climb to the top, and it is right that they should do so. The idle, infirm, and extravagant are losers, not adapted to the realities of their world, and thus legitimately subject to elimination by "social selection." Sumner presents society with an alternative: either "liberty, inequality, survival of the fittest," or "not-liberty, equality, survival of the unfittest." Self-made millionaires are the paradigm of the fittest. They are "a product of natural selection, acting on the whole body of men to pick out those who can meet the requirement of certain work to be done."

This doctrine of the financially successful as the cream of the universe naturally had a sympathetic audience. John D. Rockefeller, Andrew Carnegie, and Theodore Roosevelt were supporters, although Roosevelt believed that the unfit were entitled to some protection.

(3) Darwinism provided a rationale for Adam Smith's doctrine of the "Invisible Hand." Smith had supposed that while each man follows his innate tendency to "truck, barter, and trade," men's efforts would automatically dovetail in such a way that the economic good of society as a whole would be served. And Darwin had shown that the net result of each organism's engaging in a struggle for its own welfare was continuous evolution of the species as a whole in the direction of better adaptation to its environment. The political implications of this viewpoint are clear.

The central ethical question raised by the social Darwinists is this: granted that man is subject to natural law, and even granted further that he is subject to some form of natural or social selection, can one legitimately derive from this such policies as laissez-faire? Alfred Russel Wallace had argued that with the advent, under divine guidance, of man's brain, the evolution of man was no longer controlled by natural selection, so that inference from the doctrine of natural selection to ethical policy would be illegitimate. Huxley provided a similar argument: Man represents an island of cultural evolution in a sea of Darwinian change. These issues have largely passed into history, however, due to the philosophical point that whether or not to support a law of nature is not a question for decision.

The fate of Darwinism since the twentieth century has been mixed. Social Darwinism is of no more than historical interest. It is rightly regarded as philosophically naive and, moreover, as concerned with social questions that are not of contemporary interest. The same is largely true of the theological battles over the significance of evolution. Current theology exhibits a sublime indifference to the questions that agitated Huxley and Bishop Wilberforce. It must be pointed out, however, that modern theology is free to pursue other problems because of the clarification of the status of man and of the relation of science to theology that emerged from the Darwinian debate.

In biological theory proper, Darwin's theory remains secure. His Lamarckism is no longer accepted, if we discount some periodic revivals in the former Soviet Union; and the doctrine of sexual selection is still a matter of some debate. But the major theory of the Origin, evolution by natural selection, is the framework of modern evolutionary theory. This modern accountsometimes called the synthetic theory and sometimes, rather confusingly, Neo-Darwinismaccepts in toto the doctrine of natural selection as described above but develops it in a manner that Darwin himself could not have envisaged. The synthetic theory may fairly be described as Darwin's theory of natural selection, deepened by the absorption of twentieth-century genetics and systematically applied to the whole range of biological phenomena.

The absorption of genetics accounts for the novel developments in the doctrine of natural selection itself. Darwin thought of natural selection fundamentally as differential survival, and he regarded the organism as the natural unit that is subjected to selective pressures. With the advent of Mendelian genetics, and especially of the statistical study of the genetics of populations, these two Darwinian conceptions underwent a significant change. From the geneticist's point of view, differential survival is subordinate to differential reproduction of genetic materials; evolution is simply temporal change in the genetic constitution of a population. The simplest model of evolutionary change would be the following: Suppose that we have in a population two alleles, a 1 and a 2, of a gene a, and that a 1 is present in the proportion p, and a 2 in the proportion 1p. Then any temporal change in the value of p would be a case of reproductive differential between a 1 and a 2; and it would be an evolutionary change in the population. Some biologists simply identify such differential reproduction with natural selection, in which case sexual selection is a special case of natural selection. The natural unit of selection becomes the gene rather than the whole organism.

This conception of natural selection is not incompatible with Darwin's. Differential survival is still the major cause of differential reproduction of genes; and there is still a clear and obvious sense in which the organism is the fundamental unit of natural selection. But the new conception of natural selection facilitates the discussion of a large range of questions, for example, the roles of isolation and migration in evolution; the effectiveness of very small selective advantages; the roles of gene mutations, sex-linkage, and dominance; and so on. The modern theory has much to say on these topics that could not have been foreseen by Darwin, but nothing that he could not readily endorse.

See also Arnold, Matthew; Bergson, Henri; Darwin, Charles Robert; Darwin, Erasmus; Ethics, History of; Evolutionary Ethics; Evolutionary Theory; Good, The; Haeckel, Ernst Heinrich; Hegel, Georg Wilhelm Friedrich; Huxley, Thomas Henry; Lamarck, Chevalier de; Laws of Nature; Marx, Karl; Paley, William; Racism; Ruskin, John; Smith, Adam; Spinoza, Benedict (Baruch) de; Sumner, William Graham; Teleology; Wallace, Alfred Russel; Whitehead, Alfred North.

Darwin, Charles Robert. On the Origin of Species by means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. London, 1859. A variorum text, edited by Morse Peckham (Philadelphia, 1959), was published in paperback with an introduction by G. G. Simpson (New York, 1962). There is also a Modern Library edition (New York, 1949).

Darwin, Charles Robert. The Descent of Man, and Selection in Relation to Sex. London: J. Murray, 1871.

Darwin, Charles Robert. The Voyage of the Beagle. London and New York, n.d. A reissue by J. M. Dent and E. P. Dutton of their 1906 edition of the Journal of Researches into the Geology and Natural History of the Various Countries Visited by H. M. S. Beagle, 18321836.

Darwin, Charles Robert, and Alfred Russel Wallace. Evolution by Natural Selection. Cambridge, U.K.: Cambridge University Press, 1958. This contains Darwin's sketch of 1842, his essay of 1844, and the Darwin and Wallace papers read before the Linnaean Society in 1848.

Wallace, Alfred Russel. Contributions to the Theory of Natural Selection. London: Macmillan, 1870.

Wallace, Alfred Russel. Darwinism. London: Macmillan, 1889.

Wallace, Alfred Russel. "The Origin of Human Races and the Antiquity of Man Deduced from the Theory of Natural Selection." Journal of the Anthropological Society of London (1864).

Dewey, John. The Influence of Darwinism on Philosophy. New York, 1910.

Eiseley, Loren. Darwin's Century; Evolution and the Men Who Discovered It. Garden City, NY: Doubleday, 1958. Contains many illuminating discussions of the interplay of philosophical and scientific theories.

Fothergill, Philip. Historical Aspects of Organic Evolution. London: Hollis and Carter, 1952. A history of evolutionary theories.

Goudge, T. A. Ascent of Life; a Philosophical Study of Evolution. Toronto: University of Toronto Press, 1961.

Gray, Asa. Natural Science and Religion. New York: Scribners, 1880. A topical discussion of the way theism looked to an evolutionist.

Himmelfarb, Gertrude. Darwin and the Darwinian Revolution. Garden City, NY: Doubleday, 1959.

Hofstadter, Richard. Social Darwinism in American Thought, 18601915. Philadelphia, 1944.

Huxley, Thomas Henry. Evolution and Ethics, and Other Essays. New York, 1898.

Schneider, Herbert. "The Influence of Darwin and Spencer on American Philosophical Theology." Journal of the History of Ideas 6 (1945).

Darlington, C. D. The Evolution of Genetic Systems. Cambridge, U.K.: Cambridge University Press, 1939.

De Beer, Gavin. Embryology and Evolution. Oxford: Clarendon Press, 1930.

Dobzhansky, Theodosius. Genetics and the Origin of Species, 3rd ed. New York: Columbia University Press, 1951.

Fisher, R. A. The Genetical Theory of Natural Selection. Oxford: Clarendon Press, 1930. This is the classic application of statistical methods to the dynamics of evolving populations; together with the Darlington and Dobzhansky works, it affords a good introduction to the crucial relations between evolution and population genetics.

Mayr, Ernst. Systematics and the Origin of Species. New York: Columbia University Press, 1942.

Ross, H. H. A Synthesis of Evolutionary Theory. Englewood Cliffs, NJ: Prentice-Hall, 1962.

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