The evidence for evolution

Darwin and other 19th-century biologists found compelling evidence for biological evolution in the comparative study of living organisms, in their geographic distribution, and in the fossil remains of extinct organisms. Since Darwins time, the evidence from these sources has become considerably stronger and more comprehensive, while biological disciplines that emerged more recentlygenetics, biochemistry, physiology, ecology, animal behaviour (ethology), and especially molecular biologyhave supplied powerful additional evidence and detailed confirmation. The amount of information about evolutionary history stored in the DNA and proteins of living things is virtually unlimited; scientists can reconstruct any detail of the evolutionary history of life by investing sufficient time and laboratory resources.

Evolutionists no longer are concerned with obtaining evidence to support the fact of evolution but rather are concerned with what sorts of knowledge can be obtained from different sources of evidence. The following sections identify the most productive of these sources and illustrate the types of information they have provided.

Paleontologists have recovered and studied the fossil remains of many thousands of organisms that lived in the past. This fossil record shows that many kinds of extinct organisms were very different in form from any now living. It also shows successions of organisms through time (see faunal succession, law of; geochronology: Determining the relationships of fossils with rock strata), manifesting their transition from one form to another.

When an organism dies, it is usually destroyed by other forms of life and by weathering processes. On rare occasions some body partsparticularly hard ones such as shells, teeth, or bonesare preserved by being buried in mud or protected in some other way from predators and weather. Eventually, they may become petrified and preserved indefinitely with the rocks in which they are embedded. Methods such as radiometric datingmeasuring the amounts of natural radioactive atoms that remain in certain minerals to determine the elapsed time since they were constitutedmake it possible to estimate the time period when the rocks, and the fossils associated with them, were formed.

Radiometric dating indicates that Earth was formed about 4.5 billion years ago. The earliest fossils resemble microorganisms such as bacteria and cyanobacteria (blue-green algae); the oldest of these fossils appear in rocks 3.5 billion years old (see Precambrian time). The oldest known animal fossils, about 700 million years old, come from the so-called Ediacara fauna, small wormlike creatures with soft bodies. Numerous fossils belonging to many living phyla and exhibiting mineralized skeletons appear in rocks about 540 million years old. These organisms are different from organisms living now and from those living at intervening times. Some are so radically different that paleontologists have created new phyla in order to classify them. (See Cambrian Period.) The first vertebrates, animals with backbones, appeared about 400 million years ago; the first mammals, less than 200 million years ago. The history of life recorded by fossils presents compelling evidence of evolution.

The fossil record is incomplete. Of the small proportion of organisms preserved as fossils, only a tiny fraction have been recovered and studied by paleontologists. In some cases the succession of forms over time has been reconstructed in detail. One example is the evolution of the horse. The horse can be traced to an animal the size of a dog having several toes on each foot and teeth appropriate for browsing; this animal, called the dawn horse (genus Hyracotherium), lived more than 50 million years ago. The most recent form, the modern horse (Equus), is much larger in size, is one-toed, and has teeth appropriate for grazing. The transitional forms are well preserved as fossils, as are many other kinds of extinct horses that evolved in different directions and left no living descendants.

Using recovered fossils, paleontologists have reconstructed examples of radical evolutionary transitions in form and function. For example, the lower jaw of reptiles contains several bones, but that of mammals only one. The other bones in the reptile jaw unmistakably evolved into bones now found in the mammalian ear. At first, such a transition would seem unlikelyit is hard to imagine what function such bones could have had during their intermediate stages. Yet paleontologists discovered two transitional forms of mammal-like reptiles, called therapsids, that had a double jaw joint (i.e., two hinge points side by side)one joint consisting of the bones that persist in the mammalian jaw and the other composed of the quadrate and articular bones, which eventually became the hammer and anvil of the mammalian ear. (See also mammal: Skeleton.)

For skeptical contemporaries of Darwin, the missing linkthe absence of any known transitional form between apes and humanswas a battle cry, as it remained for uninformed people afterward. Not one but many creatures intermediate between living apes and humans have since been found as fossils. The oldest known fossil homininsi.e., primates belonging to the human lineage after it separated from lineages going to the apesare 6 million to 7 million years old, come from Africa, and are known as Sahelanthropus and Orrorin (or Praeanthropus), which were predominantly bipedal when on the ground but which had very small brains. Ardipithecus lived about 4.4 million years ago, also in Africa. Numerous fossil remains from diverse African origins are known of Australopithecus, a hominin that appeared between 3 million and 4 million years ago. Australopithecus had an upright human stance but a cranial capacity of less than 500 cc (equivalent to a brain weight of about 500 grams), comparable to that of a gorilla or a chimpanzee and about one-third that of humans. Its head displayed a mixture of ape and human characteristicsa low forehead and a long, apelike face but with teeth proportioned like those of humans. Other early hominins partly contemporaneous with Australopithecus include Kenyanthropus and Paranthropus; both had comparatively small brains, although some species of Paranthropus had larger bodies. Paranthropus represents a side branch in the hominin lineage that became extinct. Along with increased cranial capacity, other human characteristics have been found in Homo habilis, which lived about 1.5 million to 2 million years ago in Africa and had a cranial capacity of more than 600 cc (brain weight of 600 grams), and in H. erectus, which lived between 0.5 million and more than 1.5 million years ago, apparently ranged widely over Africa, Asia, and Europe, and had a cranial capacity of 800 to 1,100 cc (brain weight of 800 to 1,100 grams). The brain sizes of H. ergaster, H. antecessor, and H. heidelbergensis were roughly that of the brain of H. erectus, some of which species were partly contemporaneous, though they lived in different regions of the Eastern Hemisphere. (See also human evolution.)

The skeletons of turtles, horses, humans, birds, and bats are strikingly similar, in spite of the different ways of life of these animals and the diversity of their environments. The correspondence, bone by bone, can easily be seen not only in the limbs but also in every other part of the body. From a purely practical point of view, it is incomprehensible that a turtle should swim, a horse run, a person write, and a bird or a bat fly with forelimb structures built of the same bones. An engineer could design better limbs in each case. But if it is accepted that all of these skeletons inherited their structures from a common ancestor and became modified only as they adapted to different ways of life, the similarity of their structures makes sense.

Comparative anatomy investigates the homologies, or inherited similarities, among organisms in bone structure and in other parts of the body. The correspondence of structures is typically very close among some organismsthe different varieties of songbirds, for instancebut becomes less so as the organisms being compared are less closely related in their evolutionary history. The similarities are less between mammals and birds than they are among mammals, and they are still less between mammals and fishes. Similarities in structure, therefore, not only manifest evolution but also help to reconstruct the phylogeny, or evolutionary history, of organisms.

Comparative anatomy also reveals why most organismic structures are not perfect. Like the forelimbs of turtles, horses, humans, birds, and bats, an organisms body parts are less than perfectly adapted because they are modified from an inherited structure rather than designed from completely raw materials for a specific purpose. The imperfection of structures is evidence for evolution and against antievolutionist arguments that invoke intelligent design (see below Intelligent design and its critics).

Darwin and his followers found support for evolution in the study of embryology, the science that investigates the development of organisms from fertilized egg to time of birth or hatching. Vertebrates, from fishes through lizards to humans, develop in ways that are remarkably similar during early stages, but they become more and more differentiated as the embryos approach maturity. The similarities persist longer between organisms that are more closely related (e.g., humans and monkeys) than between those less closely related (humans and sharks). Common developmental patterns reflect evolutionary kinship. Lizards and humans share a developmental pattern inherited from their remote common ancestor; the inherited pattern of each was modified only as the separate descendant lineages evolved in different directions. The common embryonic stages of the two creatures reflect the constraints imposed by this common inheritance, which prevents changes that have not been necessitated by their diverging environments and ways of life.

The embryos of humans and other nonaquatic vertebrates exhibit gill slits even though they never breathe through gills. These slits are found in the embryos of all vertebrates because they share as common ancestors the fish in which these structures first evolved. Human embryos also exhibit by the fourth week of development a well-defined tail, which reaches maximum length at six weeks. Similar embryonic tails are found in other mammals, such as dogs, horses, and monkeys; in humans, however, the tail eventually shortens, persisting only as a rudiment in the adult coccyx.

A close evolutionary relationship between organisms that appear drastically different as adults can sometimes be recognized by their embryonic homologies. Barnacles, for example, are sedentary crustaceans with little apparent likeness to such free-swimming crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through a free-swimming larval stage, the nauplius, which is unmistakably similar to that of other crustacean larvae.

Embryonic rudiments that never fully develop, such as the gill slits in humans, are common in all sorts of animals. Some, however, like the tail rudiment in humans, persist as adult vestiges, reflecting evolutionary ancestry. The most familiar rudimentary organ in humans is the vermiform appendix. This wormlike structure attaches to a short section of intestine called the cecum, which is located at the point where the large and small intestines join. The human vermiform appendix is a functionless vestige of a fully developed organ present in other mammals, such as the rabbit and other herbivores, where a large cecum and appendix store vegetable cellulose to enable its digestion with the help of bacteria. Vestiges are instances of imperfectionslike the imperfections seen in anatomical structuresthat argue against creation by design but are fully understandable as a result of evolution.

Darwin also saw a confirmation of evolution in the geographic distribution of plants and animals, and later knowledge has reinforced his observations. For example, there are about 1,500 known species of Drosophila vinegar flies in the world; nearly one-third of them live in Hawaii and nowhere else, although the total area of the archipelago is less than one-twentieth the area of California or Germany. Also in Hawaii are more than 1,000 species of snails and other land mollusks that exist nowhere else. This unusual diversity is easily explained by evolution. The islands of Hawaii are extremely isolated and have had few colonizersi.e, animals and plants that arrived there from elsewhere and established populations. Those species that did colonize the islands found many unoccupied ecological niches, local environments suited to sustaining them and lacking predators that would prevent them from multiplying. In response, these species rapidly diversified; this process of diversifying in order to fill ecological niches is called adaptive radiation.

Each of the worlds continents has its own distinctive collection of animals and plants. In Africa are rhinoceroses, hippopotamuses, lions, hyenas, giraffes, zebras, lemurs, monkeys with narrow noses and nonprehensile tails, chimpanzees, and gorillas. South America, which extends over much the same latitudes as Africa, has none of these animals; it instead has pumas, jaguars, tapir, llamas, raccoons, opossums, armadillos, and monkeys with broad noses and large prehensile tails.

These vagaries of biogeography are not due solely to the suitability of the different environments. There is no reason to believe that South American animals are not well suited to living in Africa or those of Africa to living in South America. The islands of Hawaii are no better suited than other Pacific islands for vinegar flies, nor are they less hospitable than other parts of the world for many absent organisms. In fact, although no large mammals are native to the Hawaiian islands, pigs and goats have multiplied there as wild animals since being introduced by humans. This absence of many species from a hospitable environment in which an extraordinary variety of other species flourish can be explained by the theory of evolution, which holds that species can exist and evolve only in geographic areas that were colonized by their ancestors.

The field of molecular biology provides the most detailed and convincing evidence available for biological evolution. In its unveiling of the nature of DNA and the workings of organisms at the level of enzymes and other protein molecules, it has shown that these molecules hold information about an organisms ancestry. This has made it possible to reconstruct evolutionary events that were previously unknown and to confirm and adjust the view of events already known. The precision with which these events can be reconstructed is one reason the evidence from molecular biology is so compelling. Another reason is that molecular evolution has shown all living organisms, from bacteria to humans, to be related by descent from common ancestors.

A remarkable uniformity exists in the molecular components of organismsin the nature of the components as well as in the ways in which they are assembled and used. In all bacteria, plants, animals, and humans, the DNA comprises a different sequence of the same four component nucleotides, and all the various proteins are synthesized from different combinations and sequences of the same 20 amino acids, although several hundred other amino acids do exist. The genetic code by which the information contained in the DNA of the cell nucleus is passed on to proteins is virtually everywhere the same. Similar metabolic pathwayssequences of biochemical reactions (see metabolism)are used by the most diverse organisms to produce energy and to make up the cell components.

This unity reveals the genetic continuity and common ancestry of all organisms. There is no other rational way to account for their molecular uniformity when numerous alternative structures are equally likely. The genetic code serves as an example. Each particular sequence of three nucleotides in the nuclear DNA acts as a pattern for the production of exactly the same amino acid in all organisms. This is no more necessary than it is for a language to use a particular combination of letters to represent a particular object. If it is found that certain sequences of lettersplanet, tree, womanare used with identical meanings in a number of different books, one can be sure that the languages used in those books are of common origin.

Genes and proteins are long molecules that contain information in the sequence of their components in much the same way as sentences of the English language contain information in the sequence of their letters and words. The sequences that make up the genes are passed on from parents to offspring and are identical except for occasional changes introduced by mutations. As an illustration, one may assume that two books are being compared. Both books are 200 pages long and contain the same number of chapters. Closer examination reveals that the two books are identical page for page and word for word, except that an occasional wordsay, one in 100is different. The two books cannot have been written independently; either one has been copied from the other, or both have been copied, directly or indirectly, from the same original book. Similarly, if each component nucleotide of DNA is represented by one letter, the complete sequence of nucleotides in the DNA of a higher organism would require several hundred books of hundreds of pages, with several thousand letters on each page. When the pages (or sequences of nucleotides) in these books (organisms) are examined one by one, the correspondence in the letters (nucleotides) gives unmistakable evidence of common origin.

The two arguments presented above are based on different grounds, although both attest to evolution. Using the alphabet analogy, the first argument says that languages that use the same dictionarythe same genetic code and the same 20 amino acidscannot be of independent origin. The second argument, concerning similarity in the sequence of nucleotides in the DNA (and thus the sequence of amino acids in the proteins), says that books with very similar texts cannot be of independent origin.

The evidence of evolution revealed by molecular biology goes even farther. The degree of similarity in the sequence of nucleotides or of amino acids can be precisely quantified. For example, in humans and chimpanzees, the protein molecule called cytochrome c, which serves a vital function in respiration within cells, consists of the same 104 amino acids in exactly the same order. It differs, however, from the cytochrome c of rhesus monkeys by 1 amino acid, from that of horses by 11 additional amino acids, and from that of tuna by 21 additional amino acids. The degree of similarity reflects the recency of common ancestry. Thus, the inferences from comparative anatomy and other disciplines concerning evolutionary history can be tested in molecular studies of DNA and proteins by examining their sequences of nucleotides and amino acids. (See below DNA and protein as informational macromolecules.)

The authority of this kind of test is overwhelming; each of the thousands of genes and thousands of proteins contained in an organism provides an independent test of that organisms evolutionary history. Not all possible tests have been performed, but many hundreds have been done, and not one has given evidence contrary to evolution. There is probably no other notion in any field of science that has been as extensively tested and as thoroughly corroborated as the evolutionary origin of living organisms.

All human cultures have developed their own explanations for the origin of the world and of human beings and other creatures. Traditional Judaism and Christianity explain the origin of living beings and their adaptations to their environmentswings, gills, hands, flowersas the handiwork of an omniscient God. The philosophers of ancient Greece had their own creation myths. Anaximander proposed that animals could be transformed from one kind into another, and Empedocles speculated that they were made up of various combinations of preexisting parts. Closer to modern evolutionary ideas were the proposals of early Church Fathers such as Gregory of Nazianzus and Augustine, both of whom maintained that not all species of plants and animals were created by God; rather, some had developed in historical times from Gods creations. Their motivation was not biological but religiousit would have been impossible to hold representatives of all species in a single vessel such as Noahs Ark; hence, some species must have come into existence only after the Flood.

The notion that organisms may change by natural processes was not investigated as a biological subject by Christian theologians of the Middle Ages, but it was, usually incidentally, considered as a possibility by many, including Albertus Magnus and his student Thomas Aquinas. Aquinas concluded, after detailed discussion, that the development of living creatures such as maggots and flies from nonliving matter such as decaying meat was not incompatible with Christian faith or philosophy. But he left it to others to determine whether this actually happened.

The idea of progress, particularly the belief in unbounded human progress, was central to the Enlightenment of the 18th century, particularly in France among such philosophers as the marquis de Condorcet and Denis Diderot and such scientists as Georges-Louis Leclerc, comte de Buffon. But belief in progress did not necessarily lead to the development of a theory of evolution. Pierre-Louis Moreau de Maupertuis proposed the spontaneous generation and extinction of organisms as part of his theory of origins, but he advanced no theory of evolutioni.e., the transformation of one species into another through knowable, natural causes. Buffon, one of the greatest naturalists of the time, explicitly consideredand rejectedthe possible descent of several species from a common ancestor. He postulated that organisms arise from organic molecules by spontaneous generation, so that there could be as many kinds of animals and plants as there are viable combinations of organic molecules.

The English physician Erasmus Darwin, grandfather of Charles Darwin, offered in his Zoonomia; or, The Laws of Organic Life (179496) some evolutionary speculations, but they were not further developed and had no real influence on subsequent theories. The Swedish botanist Carolus Linnaeus devised the hierarchical system of plant and animal classification that is still in use in a modernized form. Although he insisted on the fixity of species, his classification system eventually contributed much to the acceptance of the concept of common descent.

The great French naturalist Jean-Baptiste de Monet, chevalier de Lamarck, held the enlightened view of his age that living organisms represent a progression, with humans as the highest form. From this idea he proposed, in the early years of the 19th century, the first broad theory of evolution. Organisms evolve through eons of time from lower to higher forms, a process still going on, always culminating in human beings. As organisms become adapted to their environments through their habits, modifications occur. Use of an organ or structure reinforces it; disuse leads to obliteration. The characteristics acquired by use and disuse, according to this theory, would be inherited. This assumption, later called the inheritance of acquired characteristics (or Lamarckism), was thoroughly disproved in the 20th century. Although his theory did not stand up in the light of later knowledge, Lamarck made important contributions to the gradual acceptance of biological evolution and stimulated countless later studies.

The founder of the modern theory of evolution was Charles Darwin. The son and grandson of physicians, he enrolled as a medical student at the University of Edinburgh. After two years, however, he left to study at the University of Cambridge and prepare to become a clergyman. He was not an exceptional student, but he was deeply interested in natural history. On December 27, 1831, a few months after his graduation from Cambridge, he sailed as a naturalist aboard the HMS Beagle on a round-the-world trip that lasted until October 1836. Darwin was often able to disembark for extended trips ashore to collect natural specimens.

The discovery of fossil bones from large extinct mammals in Argentina and the observation of numerous species of finches in the Galapagos Islands were among the events credited with stimulating Darwins interest in how species originate. In 1859 he published On the Origin of Species by Means of Natural Selection, a treatise establishing the theory of evolution and, most important, the role of natural selection in determining its course. He published many other books as well, notably The Descent of Man and Selection in Relation to Sex (1871), which extends the theory of natural selection to human evolution.

Darwin must be seen as a great intellectual revolutionary who inaugurated a new era in the cultural history of humankind, an era that was the second and final stage of the Copernican revolution that had begun in the 16th and 17th centuries under the leadership of men such as Nicolaus Copernicus, Galileo, and Isaac Newton. The Copernican revolution marked the beginnings of modern science. Discoveries in astronomy and physics overturned traditional conceptions of the universe. Earth no longer was seen as the centre of the universe but was seen as a small planet revolving around one of myriad stars; the seasons and the rains that make crops grow, as well as destructive storms and other vagaries of weather, became understood as aspects of natural processes; the revolutions of the planets were now explained by simple laws that also accounted for the motion of projectiles on Earth.

The significance of these and other discoveries was that they led to a conception of the universe as a system of matter in motion governed by laws of nature. The workings of the universe no longer needed to be attributed to the ineffable will of a divine Creator; rather, they were brought into the realm of sciencean explanation of phenomena through natural laws. Physical phenomena such as tides, eclipses, and positions of the planets could now be predicted whenever the causes were adequately known. Darwin accumulated evidence showing that evolution had occurred, that diverse organisms share common ancestors, and that living beings have changed drastically over the course of Earths history. More important, however, he extended to the living world the idea of nature as a system of matter in motion governed by natural laws.

Before Darwin, the origin of Earths living things, with their marvelous contrivances for adaptation, had been attributed to the design of an omniscient God. He had created the fish in the waters, the birds in the air, and all sorts of animals and plants on the land. God had endowed these creatures with gills for breathing, wings for flying, and eyes for seeing, and he had coloured birds and flowers so that human beings could enjoy them and recognize Gods wisdom. Christian theologians, from Aquinas on, had argued that the presence of design, so evident in living beings, demonstrates the existence of a supreme Creator; the argument from design was Aquinass fifth way for proving the existence of God. In 19th-century England the eight Bridgewater Treatises were commissioned so that eminent scientists and philosophers would expand on the marvels of the natural world and thereby set forth the Power, wisdom, and goodness of God as manifested in the Creation.

The British theologian William Paley in his Natural Theology (1802) used natural history, physiology, and other contemporary knowledge to elaborate the argument from design. If a person should find a watch, even in an uninhabited desert, Paley contended, the harmony of its many parts would force him to conclude that it had been created by a skilled watchmaker; and, Paley went on, how much more intricate and perfect in design is the human eye, with its transparent lens, its retina placed at the precise distance for forming a distinct image, and its large nerve transmitting signals to the brain.

The argument from design seems to be forceful. A ladder is made for climbing, a knife for cutting, and a watch for telling time; their functional design leads to the conclusion that they have been fashioned by a carpenter, a smith, or a watchmaker. Similarly, the obvious functional design of animals and plants seems to denote the work of a Creator. It was Darwins genius that he provided a natural explanation for the organization and functional design of living beings. (For additional discussion of the argument from design and its revival in the 1990s, see below Intelligent design and its critics.)

Darwin accepted the facts of adaptationhands are for grasping, eyes for seeing, lungs for breathing. But he showed that the multiplicity of plants and animals, with their exquisite and varied adaptations, could be explained by a process of natural selection, without recourse to a Creator or any designer agent. This achievement would prove to have intellectual and cultural implications more profound and lasting than his multipronged evidence that convinced contemporaries of the fact of evolution.

Darwins theory of natural selection is summarized in the Origin of Species as follows:

As many more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life.Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.

Natural selection was proposed by Darwin primarily to account for the adaptive organization of living beings; it is a process that promotes or maintains adaptation. Evolutionary change through time and evolutionary diversification (multiplication of species) are not directly promoted by natural selection, but they often ensue as by-products of natural selection as it fosters adaptation to different environments.

The publication of the Origin of Species produced considerable public excitement. Scientists, politicians, clergymen, and notables of all kinds read and discussed the book, defending or deriding Darwins ideas. The most visible actor in the controversies immediately following publication was the English biologist T.H. Huxley, known as Darwins bulldog, who defended the theory of evolution with articulate and sometimes mordant words on public occasions as well as in numerous writings. Evolution by natural selection was indeed a favourite topic in society salons during the 1860s and beyond. But serious scientific controversies also arose, first in Britain and then on the Continent and in the United States.

One occasional participant in the discussion was the British naturalist Alfred Russel Wallace, who had hit upon the idea of natural selection independently and had sent a short manuscript about it to Darwin from the Malay Archipelago, where he was collecting specimens and writing. On July 1, 1858, one year before the publication of the Origin, a paper jointly authored by Wallace and Darwin was presented, in the absence of both, to the Linnean Society in Londonwith apparently little notice. Greater credit is duly given to Darwin than to Wallace for the idea of evolution by natural selection; Darwin developed the theory in considerably more detail, provided far more evidence for it, and was primarily responsible for its acceptance. Wallaces views differed from Darwins in several ways, most importantly in that Wallace did not think natural selection sufficient to account for the origin of human beings, which in his view required direct divine intervention.

A younger English contemporary of Darwin, with considerable influence during the latter part of the 19th and in the early 20th century, was Herbert Spencer. A philosopher rather than a biologist, he became an energetic proponent of evolutionary ideas, popularized a number of slogans, such as survival of the fittest (which was taken up by Darwin in later editions of the Origin), and engaged in social and metaphysical speculations. His ideas considerably damaged proper understanding and acceptance of the theory of evolution by natural selection. Darwin wrote of Spencers speculations:

His deductive manner of treating any subject is wholly opposed to my frame of mind.His fundamental generalizations (which have been compared in importance by some persons with Newtons laws!) which I dare say may be very valuable under a philosophical point of view, are of such a nature that they do not seem to me to be of any strictly scientific use.

Most pernicious was the crude extension by Spencer and others of the notion of the struggle for existence to human economic and social life that became known as social Darwinism (see below Scientific acceptance and extension to other disciplines).

The most serious difficulty facing Darwins evolutionary theory was the lack of an adequate theory of inheritance that would account for the preservation through the generations of the variations on which natural selection was supposed to act. Contemporary theories of blending inheritance proposed that offspring merely struck an average between the characteristics of their parents. But as Darwin became aware, blending inheritance (including his own theory of pangenesis, in which each organ and tissue of an organism throws off tiny contributions of itself that are collected in the sex organs and determine the configuration of the offspring) could not account for the conservation of variations, because differences between variant offspring would be halved each generation, rapidly reducing the original variation to the average of the preexisting characteristics.

The missing link in Darwins argument was provided by Mendelian genetics. About the time the Origin of Species was published, the Augustinian monk Gregor Mendel was starting a long series of experiments with peas in the garden of his monastery in Brnn, Austria-Hungary (now Brno, Czech Republic). These experiments and the analysis of their results are by any standard an example of masterly scientific method. Mendels paper, published in 1866 in the Proceedings of the Natural Science Society of Brnn, formulated the fundamental principles of the theory of heredity that is still current. His theory accounts for biological inheritance through particulate factors (now known as genes) inherited one from each parent, which do not mix or blend but segregate in the formation of the sex cells, or gametes.

Mendels discoveries remained unknown to Darwin, however, and, indeed, they did not become generally known until 1900, when they were simultaneously rediscovered by a number of scientists on the Continent. In the meantime, Darwinism in the latter part of the 19th century faced an alternative evolutionary theory known as neo-Lamarckism. This hypothesis shared with Lamarcks the importance of use and disuse in the development and obliteration of organs, and it added the notion that the environment acts directly on organic structures, which explained their adaptation to the way of life and environment of the organism. Adherents of this theory discarded natural selection as an explanation for adaptation to the environment.

Prominent among the defenders of natural selection was the German biologist August Weismann, who in the 1880s published his germ plasm theory. He distinguished two substances that make up an organism: the soma, which comprises most body parts and organs, and the germ plasm, which contains the cells that give rise to the gametes and hence to progeny. Early in the development of an egg, the germ plasm becomes segregated from the somatic cells that give rise to the rest of the body. This notion of a radical separation between germ plasm and somathat is, between the reproductive tissues and all other body tissuesprompted Weismann to assert that inheritance of acquired characteristics was impossible, and it opened the way for his championship of natural selection as the only major process that would account for biological evolution. Weismanns ideas became known after 1896 as neo-Darwinism.

The rediscovery in 1900 of Mendels theory of heredity, by the Dutch botanist and geneticist Hugo de Vries and others, led to an emphasis on the role of heredity in evolution. De Vries proposed a new theory of evolution known as mutationism, which essentially did away with natural selection as a major evolutionary process. According to de Vries (who was joined by other geneticists such as William Bateson in England), two kinds of variation take place in organisms. One is the ordinary variability observed among individuals of a species, which is of no lasting consequence in evolution because, according to de Vries, it could not lead to a transgression of the species border [i.e., to establishment of new species] even under conditions of the most stringent and continued selection. The other consists of the changes brought about by mutations, spontaneous alterations of genes that result in large modifications of the organism and give rise to new species: The new species thus originates suddenly, it is produced by the existing one without any visible preparation and without transition.

Mutationism was opposed by many naturalists and in particular by the so-called biometricians, led by the English statistician Karl Pearson, who defended Darwinian natural selection as the major cause of evolution through the cumulative effects of small, continuous, individual variations (which the biometricians assumed passed from one generation to the next without being limited by Mendels laws of inheritance [see Mendelism]).

The controversy between mutationists (also referred to at the time as Mendelians) and biometricians approached a resolution in the 1920s and 30s through the theoretical work of geneticists. These scientists used mathematical arguments to show, first, that continuous variation (in such characteristics as body size, number of eggs laid, and the like) could be explained by Mendels laws and, second, that natural selection acting cumulatively on small variations could yield major evolutionary changes in form and function. Distinguished members of this group of theoretical geneticists were R.A. Fisher and J.B.S. Haldane in Britain and Sewall Wright in the United States. Their work contributed to the downfall of mutationism and, most important, provided a theoretical framework for the integration of genetics into Darwins theory of natural selection. Yet their work had a limited impact on contemporary biologists for several reasonsit was formulated in a mathematical language that most biologists could not understand; it was almost exclusively theoretical, with little empirical corroboration; and it was limited in scope, largely omitting many issues, such as speciation (the process by which new species are formed), that were of great importance to evolutionists.

A major breakthrough came in 1937 with the publication of Genetics and the Origin of Species by Theodosius Dobzhansky, a Russian-born American naturalist and experimental geneticist. Dobzhanskys book advanced a reasonably comprehensive account of the evolutionary process in genetic terms, laced with experimental evidence supporting the theoretical argument. Genetics and the Origin of Species may be considered the most important landmark in the formulation of what came to be known as the synthetic theory of evolution, effectively combining Darwinian natural selection and Mendelian genetics. It had an enormous impact on naturalists and experimental biologists, who rapidly embraced the new understanding of the evolutionary process as one of genetic change in populations. Interest in evolutionary studies was greatly stimulated, and contributions to the theory soon began to follow, extending the synthesis of genetics and natural selection to a variety of biological fields.

The main writers who, together with Dobzhansky, may be considered the architects of the synthetic theory were the German-born American zoologist Ernst Mayr, the English zoologist Julian Huxley, the American paleontologist George Gaylord Simpson, and the American botanist George Ledyard Stebbins. These researchers contributed to a burst of evolutionary studies in the traditional biological disciplines and in some emerging onesnotably population genetics and, later, evolutionary ecology (see community ecology). By 1950 acceptance of Darwins theory of evolution by natural selection was universal among biologists, and the synthetic theory had become widely adopted.

The most important line of investigation after 1950 was the application of molecular biology to evolutionary studies. In 1953 the American geneticist James Watson and the British biophysicist Francis Crick deduced the molecular structure of DNA (deoxyribonucleic acid), the hereditary material contained in the chromosomes of every cells nucleus. The genetic information is encoded within the sequence of nucleotides that make up the chainlike DNA molecules. This information determines the sequence of amino acid building blocks of protein molecules, which include, among others, structural proteins such as collagen, respiratory proteins such as hemoglobin, and numerous enzymes responsible for the organisms fundamental life processes. Genetic information contained in the DNA can thus be investigated by examining the sequences of amino acids in the proteins.

In the mid-1960s laboratory techniques such as electrophoresis and selective assay of enzymes became available for the rapid and inexpensive study of differences among enzymes and other proteins. The application of these techniques to evolutionary problems made possible the pursuit of issues that earlier could not be investigatedfor example, exploring the extent of genetic variation in natural populations (which sets bounds on their evolutionary potential) and determining the amount of genetic change that occurs during the formation of new species.

Comparisons of the amino acid sequences of corresponding proteins in different species provided quantitatively precise measures of the divergence among species evolved from common ancestors, a considerable improvement over the typically qualitative evaluations obtained by comparative anatomy and other evolutionary subdisciplines. In 1968 the Japanese geneticist Motoo Kimura proposed the neutrality theory of molecular evolution, which assumes that, at the level of the sequences of nucleotides in DNA and of amino acids in proteins, many changes are adaptively neutral; they have little or no effect on the molecules function and thus on an organisms fitness within its environment. If the neutrality theory is correct, there should be a molecular clock of evolution; that is, the degree to which amino acid or nucleotide sequences diverge between species should provide a reliable estimate of the time since the species diverged. This would make it possible to reconstruct an evolutionary history that would reveal the order of branching of different lineages, such as those leading to humans, chimpanzees, and orangutans, as well as the time in the past when the lineages split from one another. During the 1970s and 80s it gradually became clear that the molecular clock is not exact; nevertheless, into the early 21st century it continued to provide the most reliable evidence for reconstructing evolutionary history. (See below The molecular clock of evolution and The neutrality theory of molecular evolution.)

The laboratory techniques of DNA cloning and sequencing have provided a new and powerful means of investigating evolution at the molecular level. The fruits of this technology began to accumulate during the 1980s following the development of automated DNA-sequencing machines and the invention of the polymerase chain reaction (PCR), a simple and inexpensive technique that obtains, in a few hours, billions or trillions of copies of a specific DNA sequence or gene. Major research efforts such as the Human Genome Project further improved the technology for obtaining long DNA sequences rapidly and inexpensively. By the first few years of the 21st century, the full DNA sequencei.e., the full genetic complement, or genomehad been obtained for more than 20 higher organisms, including human beings, the house mouse (Mus musculus), the rat Rattus norvegicus, the vinegar fly Drosophila melanogaster, the mosquito Anopheles gambiae, the nematode worm Caenorhabditis elegans, the malaria parasite Plasmodium falciparum, the mustard weed Arabidopsis thaliana, and the yeast Saccharomyces cerevisiae, as well as for numerous microorganisms. Additional research during this time explored alternative mechanisms of inheritance, including epigenetic modification (the chemical modification of specific genes or gene-associated proteins), that could explain an organisms ability to transmit traits developed during its lifetime to its offspring.

The Earth sciences also experienced, in the second half of the 20th century, a conceptual revolution with considerable consequence to the study of evolution. The theory of plate tectonics, which was formulated in the late 1960s, revealed that the configuration and position of the continents and oceans are dynamic, rather than static, features of Earth. Oceans grow and shrink, while continents break into fragments or coalesce into larger masses. The continents move across Earths surface at rates of a few centimetres a year, and over millions of years of geologic history this movement profoundly alters the face of the planet, causing major climatic changes along the way. These previously unsuspected massive modifications of Earths past environments are, of necessity, reflected in the evolutionary history of life. Biogeography, the evolutionary study of plant and animal distribution, has been revolutionized by the knowledge, for example, that Africa and South America were part of a single landmass some 200 million years ago and that the Indian subcontinent was not connected with Asia until geologically recent times.

Ecology, the study of the interactions of organisms with their environments, has evolved from descriptive studiesnatural historyinto a vigorous biological discipline with a strong mathematical component, both in the development of theoretical models and in the collection and analysis of quantitative data. Evolutionary ecology (see community ecology) is an active field of evolutionary biology; another is evolutionary ethology, the study of the evolution of animal behaviour. Sociobiology, the evolutionary study of social behaviour, is perhaps the most active subfield of ethology. It is also the most controversial, because of its extension to human societies.

The theory of evolution makes statements about three different, though related, issues: (1) the fact of evolutionthat is, that organisms are related by common descent; (2) evolutionary historythe details of when lineages split from one another and of the changes that occurred in each lineage; and (3) the mechanisms or processes by which evolutionary change occurs.

The first issue is the most fundamental and the one established with utmost certainty. Darwin gathered much evidence in its support, but evidence has accumulated continuously ever since, derived from all biological disciplines. The evolutionary origin of organisms is today a scientific conclusion established with the kind of certainty attributable to such scientific concepts as the roundness of Earth, the motions of the planets, and the molecular composition of matter. This degree of certainty beyond reasonable doubt is what is implied when biologists say that evolution is a fact; the evolutionary origin of organisms is accepted by virtually every biologist.

But the theory of evolution goes far beyond the general affirmation that organisms evolve. The second and third issuesseeking to ascertain evolutionary relationships between particular organisms and the events of evolutionary history, as well as to explain how and why evolution takes placeare matters of active scientific investigation. Some conclusions are well established. One, for example, is that the chimpanzee and the gorilla are more closely related to humans than is any of those three species to the baboon or other monkeys. Another conclusion is that natural selection, the process postulated by Darwin, explains the configuration of such adaptive features as the human eye and the wings of birds. Many matters are less certain, others are conjectural, and still otherssuch as the characteristics of the first living things and when they came aboutremain completely unknown.

Since Darwin, the theory of evolution has gradually extended its influence to other biological disciplines, from physiology to ecology and from biochemistry to systematics. All biological knowledge now includes the phenomenon of evolution. In the words of Theodosius Dobzhansky, Nothing in biology makes sense except in the light of evolution.

The term evolution and the general concept of change through time also have penetrated into scientific language well beyond biology and even into common language. Astrophysicists speak of the evolution of the solar system or of the universe; geologists, of the evolution of Earths interior; psychologists, of the evolution of the mind; anthropologists, of the evolution of cultures; art historians, of the evolution of architectural styles; and couturiers, of the evolution of fashion. These and other disciplines use the word with only the slightest commonality of meaningthe notion of gradual, and perhaps directional, change over the course of time.

Toward the end of the 20th century, specific concepts and processes borrowed from biological evolution and living systems were incorporated into computational research, beginning with the work of the American mathematician John Holland and others. One outcome of this endeavour was the development of methods for automatically generating computer-based systems that are proficient at given tasks. These systems have a wide variety of potential uses, such as solving practical computational problems, providing machines with the ability to learn from experience, and modeling processes in fields as diverse as ecology, immunology, economics, and even biological evolution itself.

To generate computer programs that represent proficient solutions to a problem under study, the computer scientist creates a set of step-by-step procedures, called a genetic algorithm or, more broadly, an evolutionary algorithm, that incorporates analogies of genetic processesfor instance, heredity, mutation, and recombinationas well as of evolutionary processes such as natural selection in the presence of specified environments. The algorithm is designed typically to simulate the biological evolution of a population of individual computer programs through successive generations to improve their fitness for carrying out a designated task. Each program in an initial population receives a fitness score that measures how well it performs in a specific environmentfor example, how efficiently it sorts a list of numbers or allocates the floor space in a new factory design. Only those with the highest scores are selected to reproduce, to contribute hereditary materiali.e., computer codeto the following generation of programs. The rules of reproduction may involve such elements as recombination (strings of code from the best programs are shuffled and combined into the programs of the next generation) and mutation (bits of code in a few of the new programs are changed at random). The evolutionary algorithm then evaluates each program in the new generation for fitness, winnows out the poorer performers, and allows reproduction to take place once again, with the cycle repeating itself as often as desired. Evolutionary algorithms are simplistic compared with biological evolution, but they have provided robust and powerful mechanisms for finding solutions to all sorts of problems in economics, industrial production, and the distribution of goods and services. (See also artificial intelligence: Evolutionary computing.)

Darwins notion of natural selection also has been extended to areas of human discourse outside the scientific setting, particularly in the fields of sociopolitical theory and economics. The extension can be only metaphoric, because in Darwins intended meaning natural selection applies only to hereditary variations in entities endowed with biological reproductionthat is, to living organisms. That natural selection is a natural process in the living world has been taken by some as a justification for ruthless competition and for survival of the fittest in the struggle for economic advantage or for political hegemony. Social Darwinism was an influential social philosophy in some circles through the late 19th and early 20th centuries, when it was used as a rationalization for racism, colonialism, and social stratification. At the other end of the political spectrum, Marxist theorists have resorted to evolution by natural selection as an explanation for humankinds political history.

Darwinism understood as a process that favours the strong and successful and eliminates the weak and failing has been used to justify alternative and, in some respects, quite diametric economic theories (see economics). These theories share in common the premise that the valuation of all market products depends on a Darwinian process. Specific market commodities are evaluated in terms of the degree to which they conform to specific valuations emanating from the consumers. On the one hand, some of these economic theories are consistent with theories of evolutionary psychology that see preferences as determined largely genetically; as such, they hold that the reactions of markets can be predicted in terms of largely fixed human attributes. The dominant neo-Keynesian (see economics: Keynesian economics) and monetarist schools of economics make predictions of the macroscopic behaviour of economies (see macroeconomics) based the interrelationship of a few variables; money supply, rate of inflation, and rate of unemployment jointly determine the rate of economic growth. On the other hand, some minority economists, such as the 20th-century Austrian-born British theorist F.A. Hayek and his followers, predicate the Darwinian process on individual preferences that are mostly underdetermined and change in erratic or unpredictable ways. According to them, old ways of producing goods and services are continuously replaced by new inventions and behaviours. These theorists affirm that what drives the economy is the ingenuity of individuals and corporations and their ability to bring new and better products to the market.

The theory of evolution has been seen by some people as incompatible with religious beliefs, particularly those of Christianity. The first chapters of the biblical book of Genesis describe Gods creation of the world, the plants, the animals, and human beings. A literal interpretation of Genesis seems incompatible with the gradual evolution of humans and other organisms by natural processes. Independently of the biblical narrative, the Christian beliefs in the immortality of the soul and in humans as created in the image of God have appeared to many as contrary to the evolutionary origin of humans from nonhuman animals.

Religiously motivated attacks started during Darwins lifetime. In 1874 Charles Hodge, an American Protestant theologian, published What Is Darwinism?, one of the most articulate assaults on evolutionary theory. Hodge perceived Darwins theory as the most thoroughly naturalistic that can be imagined and far more atheistic than that of his predecessor Lamarck. He argued that the design of the human eye evinces that it has been planned by the Creator, like the design of a watch evinces a watchmaker. He concluded that the denial of design in nature is actually the denial of God.

Other Protestant theologians saw a solution to the difficulty through the argument that God operates through intermediate causes. The origin and motion of the planets could be explained by the law of gravity and other natural processes without denying Gods creation and providence. Similarly, evolution could be seen as the natural process through which God brought living beings into existence and developed them according to his plan. Thus, A.H. Strong, the president of Rochester Theological Seminary in New York state, wrote in his Systematic Theology (1885): We grant the principle of evolution, but we regard it as only the method of divine intelligence. The brutish ancestry of human beings was not incompatible with their excelling status as creatures in the image of God. Strong drew an analogy with Christs miraculous conversion of water into wine: The wine in the miracle was not water because water had been used in the making of it, nor is man a brute because the brute has made some contributions to its creation. Arguments for and against Darwins theory came from Roman Catholic theologians as well.

Gradually, well into the 20th century, evolution by natural selection came to be accepted by the majority of Christian writers. Pope Pius XII in his encyclical Humani generis (1950; Of the Human Race) acknowledged that biological evolution was compatible with the Christian faith, although he argued that Gods intervention was necessary for the creation of the human soul. Pope John Paul II, in an address to the Pontifical Academy of Sciences on October 22, 1996, deplored interpreting the Bibles texts as scientific statements rather than religious teachings, adding:

New scientific knowledge has led us to realize that the theory of evolution is no longer a mere hypothesis. It is indeed remarkable that this theory has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge. The convergence, neither sought nor fabricated, of the results of work that was conducted independently is in itself a significant argument in favor of this theory.

Similar views were expressed by other mainstream Christian denominations. The General Assembly of the United Presbyterian Church in 1982 adopted a resolution stating that Biblical scholars and theological schoolsfind that the scientific theory of evolution does not conflict with their interpretation of the origins of life found in Biblical literature. The Lutheran World Federation in 1965 affirmed that evolutions assumptions are as much around us as the air we breathe and no more escapable. At the same time theologys affirmations are being made as responsibly as ever. In this sense both science and religion are here to stay, andneed to remain in a healthful tension of respect toward one another. Similar statements have been advanced by Jewish authorities and those of other major religions. In 1984 the 95th Annual Convention of the Central Conference of American Rabbis adopted a resolution stating: Whereas the principles and concepts of biological evolution are basic to understanding sciencewe call upon science teachers and local school authorities in all states to demand quality textbooks that are based on modern, scientific knowledge and that exclude scientific creationism.

Opposing these views were Christian denominations that continued to hold a literal interpretation of the Bible. A succinct expression of this interpretation is found in the Statement of Belief of the Creation Research Society, founded in 1963 as a professional organization of trained scientists and interested laypersons who are firmly committed to scientific special creation (see creationism):

The Bible is the Written Word of God, and because it is inspired throughout, all of its assertions are historically and scientifically true in the original autographs. To the student of nature this means that the account of origins in Genesis is a factual presentation of simple historical truths.

Many Bible scholars and theologians have long rejected a literal interpretation as untenable, however, because the Bible contains incompatible statements. The very beginning of the book of Genesis presents two different creation narratives. Extending through chapter 1 and the first verses of chapter 2 is the familiar six-day narrative, in which God creates human beingsboth male and femalein his own image on the sixth day, after creating light, Earth, firmament, fish, fowl, and cattle. But in verse 4 of chapter 2 a different narrative starts, in which God creates a male human, then plants a garden and creates the animals, and only then proceeds to take a rib from the man to make a woman.

Biblical scholars point out that the Bible is inerrant with respect to religious truth, not in matters that are of no significance to salvation. Augustine, considered by many the greatest Christian theologian, wrote in the early 5th century in his De Genesi ad litteram (Literal Commentary on Genesis):

It is also frequently asked what our belief must be about the form and shape of heaven, according to Sacred Scripture. Many scholars engage in lengthy discussions on these matters, but the sacred writers with their deeper wisdom have omitted them. Such subjects are of no profit for those who seek beatitude. And what is worse, they take up very precious time that ought to be given to what is spiritually beneficial. What concern is it of mine whether heaven is like a sphere and Earth is enclosed by it and suspended in the middle of the universe, or whether heaven is like a disk and the Earth is above it and hovering to one side.

Augustine adds later in the same chapter: In the matter of the shape of heaven, the sacred writers did not wish to teach men facts that could be of no avail for their salvation. Augustine is saying that the book of Genesis is not an elementary book of astronomy. It is a book about religion, and it is not the purpose of its religious authors to settle questions about the shape of the universe that are of no relevance whatsoever to how to seek salvation.

In the same vein, John Paul II said in 1981:

The Bible itself speaks to us of the origin of the universe and its make-up, not in order to provide us with a scientific treatise but in order to state the correct relationships of man with God and with the universe. Sacred scripture wishes simply to declare that the world was created by God, and in order to teach this truth it expresses itself in the terms of the cosmology in use at the time of the writer.Any other teaching about the origin and make-up of the universe is alien to the intentions of the Bible, which does not wish to teach how the heavens were made but how one goes to heaven.

John Pauls argument was clearly a response to Christian fundamentalists who see in Genesis a literal description of how the world was created by God. In modern times biblical fundamentalists have made up a minority of Christians, but they have periodically gained considerable public and political influence, particularly in the United States. Opposition to the teaching of evolution in the United States can largely be traced to two movements with 19th-century roots, Seventh-day Adventism (see Adventist) and Pentecostalism. Consistent with their emphasis on the seventh-day Sabbath as a memorial of the biblical Creation, Seventh-day Adventists have insisted on the recent creation of life and the universality of the Flood, which they believe deposited the fossil-bearing rocks. This distinctively Adventist interpretation of Genesis became the hard core of creation science in the late 20th century and was incorporated into the balanced-treatment laws of Arkansas and Louisiana (discussed below). Many Pentecostals, who generally endorse a literal interpretation of the Bible, also have adopted and endorsed the tenets of creation science, including the recent origin of Earth and a geology interpreted in terms of the Flood. They have differed from Seventh-day Adventists and other adherents of creation science, however, in their tolerance of diverse views and the limited import they attribute to the evolution-creation controversy.

During the 1920s, biblical fundamentalists helped influence more than 20 state legislatures to debate antievolution laws, and four statesArkansas, Mississippi, Oklahoma, and Tennesseeprohibited the teaching of evolution in their public schools. A spokesman for the antievolutionists was William Jennings Bryan, three times the unsuccessful Democratic candidate for the U.S. presidency, who said in 1922, We will drive Darwinism from our schools. In 1925 Bryan took part in the prosecution (see Scopes Trial) of John T. Scopes, a high-school teacher in Dayton, Tennessee, who had admittedly violated the states law forbidding the teaching of evolution.

In 1968 the Supreme Court of the United States declared unconstitutional any law banning the teaching of evolution in public schools. After that time Christian fundamentalists introduced bills in a number of state legislatures ordering that the teaching of evolution science be balanced by allocating equal time to creation science. Creation science maintains that all kinds of organisms abruptly came into existence when God created the universe, that the world is only a few thousand years old, and that the biblical Flood was an actual event that only one pair of each animal species survived. In the 1980s Arkansas and Louisiana passed acts requiring the balanced treatment of evolution science and creation science in their schools, but opponents successfully challenged the acts as violations of the constitutionally mandated separation of church and state. The Arkansas statute was declared unconstitutional in federal court after a public trial in Little Rock. The Louisiana law was appealed all the way to the Supreme Court of the United States, which ruled Louisianas Creationism Act unconstitutional because, by advancing the religious belief that a supernatural being created humankind, which is embraced by the phrase creation science, the act impermissibly endorses religion.

William Paleys Natural Theology, the book by which he has become best known to posterity, is a sustained argument explaining the obvious design of humans and their parts, as well as the design of all sorts of organisms, in themselves and in their relations to one another and to their environment. Paleys keystone claim is that there cannot be design without a designer; contrivance, without a contriver; order, without choice;means suitable to an end, and executing their office in accomplishing that end, without the end ever having been contemplated. His book has chapters dedicated to the complex design of the human eye; to the human frame, which, he argues, displays a precise mechanical arrangement of bones, cartilage, and joints; to the circulation of the blood and the disposition of blood vessels; to the comparative anatomy of humans and animals; to the digestive system, kidneys, urethra, and bladder; to the wings of birds and the fins of fish; and much more. For more than 300 pages, Paley conveys extensive and accurate biological knowledge in such detail and precision as was available in 1802, the year of the books publication. After his meticulous description of each biological object or process, Paley draws again and again the same conclusiononly an omniscient and omnipotent deity could account for these marvels and for the enormous diversity of inventions that they entail.

On the example of the human eye he wrote:

I know no better method of introducing so large a subject, than that of comparingan eye, for example, with a telescope. As far as the examination of the instrument goes, there is precisely the same proof that the eye was made for vision, as there is that the telescope was made for assisting it. They are made upon the same principles; both being adjusted to the laws by which the transmission and refraction of rays of light are regulated.For instance, these laws require, in order to produce the same effect, that the rays of light, in passing from water into the eye, should be refracted by a more convex surface than when it passes out of air into the eye. Accordingly we find that the eye of a fish, in that part of it called the crystalline lens, is much rounder than the eye of terrestrial animals. What plainer manifestation of design can there be than this difference? What could a mathematical instrument maker have done more to show his knowledge of [t]his principle, his application of that knowledge, his suiting of his means to his endto testify counsel, choice, consideration, purpose?

It would be absurd to suppose, he argued, that by mere chance the eye

should have consisted, first, of a series of transparent lensesvery different, by the by, even in their substance, from the opaque materials of which the rest of the body is, in general at least, composed, and with which the whole of its surface, this single portion of it excepted, is covered: secondly, of a black cloth or canvasthe only membrane in the body which is blackspread out behind these lenses, so as to receive the image formed by pencils of light transmitted through them; and placed at the precise geometrical distance at which, and at which alone, a distinct image could be formed, namely, at the concourse of the refracted rays: thirdly, of a large nerve communicating between this membrane and the brain; without which, the action of light upon the membrane, however modified by the organ, would be lost to the purposes of sensation.

The strength of the argument against chance derived, according to Paley, from a notion that he named relation and that later authors would term irreducible complexity. Paley wrote:

When several different parts contribute to one effect, or, which is the same thing, when an effect is produced by the joint action of different instruments, the fitness of such parts or instruments to one another for the purpose of producing, by their united action, the effect, is what I call relation; and wherever this is observed in the works of nature or of man, it appears to me to carry along with it decisive evidence of understanding, intention, artall depending upon the motions within, all upon the system of intermediate actions.

Natural Theology was part of the canon at Cambridge for half a century after Paleys death. It thus was read by Darwin, who was an undergraduate student there between 1827 and 1831, with profit and much delight. Darwin was mindful of Paleys relation argument when in the Origin of Species he stated: If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case.We should be extremely cautious in concluding that an organ could not have been formed by transitional gradations of some kind.

In the 1990s several authors revived the argument from design. The proposition, once again, was that living beings manifest intelligent designthey are so diverse and complicated that they can be explained not as the outcome of natural processes but only as products of an intelligent designer. Some authors clearly equated this entity with the omnipotent God of Christianity and other monotheistic religions. Others, because they wished to see the theory of intelligent design taught in schools as an alternate to the theory of evolution, avoided all explicit reference to God in order to maintain the separation between religion and state.

The call for an intelligent designer is predicated on the existence of irreducible complexity in organisms. In Michael Behes book Darwins Black Box: The Biochemical Challenge to Evolution (1996), an irreducibly complex system is defined as being composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. Contemporary intelligent-design proponents have argued that irreducibly complex systems cannot be the outcome of evolution. According to Behe, Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on. In other words, unless all parts of the eye come simultaneously into existence, the eye cannot function; it does not benefit a precursor organism to have just a retina, or a lens, if the other parts are lacking. The human eye, they conclude, could not have evolved one small step at a time, in the piecemeal manner by which natural selection works.

The theory of intelligent design has encountered many critics, not only among evolutionary scientists but also among theologians and religious authors. Evolutionists point out that organs and other components of living beings are not irreducibly complexthey do not come about suddenly, or in one fell swoop. The human eye did not appear suddenly in all its present complexity. Its formation required the integration of many genetic units, each improving the performance of preexisting, functionally less-perfect eyes. About 700 million years ago, the ancestors of todays vertebrates already had organs sensitive to light. Mere perception of lightand, later, various levels of vision abilitywere beneficial to these organisms living in environments pervaded by sunlight. As is discussed more fully below in the section Diversity and extinction, different kinds of eyes have independently evolved at least 40 times in animals, which exhibit a full range, from very uncomplicated modifications that allow individual cells or simple animals to perceive the direction of light to the sophisticated vertebrate eye, passing through all sorts of organs intermediate in complexity. Evolutionists have shown that the examples of irreducibly complex systems cited by intelligent-design theoristssuch as the biochemical mechanism of blood clotting (see coagulation) or the molecular rotary motor, called the flagellum, by which bacterial cells moveare not irreducible at all; rather, less-complex versions of the same systems can be found in todays organisms.

Evolutionists have pointed out as well that imperfections and defects pervade the living world. In the human eye, for example, the visual nerve fibres in the eye converge on an area of the retina to form the optic nerve and thus create a blind spot; squids and octopuses do not have this defect. Defective design seems incompatible with an omnipotent intelligent designer. Anticipating this criticism, Paley responded that apparent blemishesought to be referred to some cause, though we be ignorant of it. Modern intelligent-design theorists have made similar assertions; according to Behe, The argument from imperfection overlooks the possibility that the designer might have multiple motives, with engineering excellence oftentimes relegated to a secondary role. This statement, evolutionists have responded, may have theological validity, but it destroys intelligent design as a scientific hypothesis, because it provides it with an empirically impenetrable shield against predictions of how intelligent or perfect a design will be. Science tests its hypotheses by observing whether predictions derived from them are the case in the observable world. A hypothesis that cannot be tested empiricallythat is, by observation or experimentis not scientific. The implication of this line of reasoning for U.S. public schools has been recognized not only by scientists but also by nonscientists, including politicians and policy makers. The liberal U.S. senator Edward Kennedy wrote in 2002 that intelligent design is not a genuine scientific theory and, therefore, has no place in the curriculum of our nations public school science classes.

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