The Semantic Theory of Evolution,

Marcello Barbieri
CHAPTER 1

The Idea of Evolution

THE IDEA OF CHANGE

Every schoolboy--wrote Arthur Lovejoy in 1909--knows that Darwin did not invent the theory of evolution, and the general public is more or less acquainted with the names and the works of at least some of the earlier protagonists of the doctrine: of Darwin's grandfather Erasmus, of Lamarck, of Geoffroy St. Hilaire, and of Herbert Spencer.

This point has been emphasized by various authors ever since. In 1932, for example, J.B.S. Haldane expressed it in this way:

We must carefully distinguish between two quite different doctrines which Darwin popularized, the doctrine of evolution and that of natural selection. It is quite possible to hold the first and not the second.

All that remains, it seems, is the idea of change. In 1888, Joseph LeConte summarized a widespread opinion in the following way:<

Evolution is not a special theory--Lamarckian, Darwinian, Spencerian--for these are all more or less successful modes of explaining it. Evolution is a law of derivation of forms from previous forms, a law of continuity, a universal law of becoming. It is only necessary to conceive it clearly, to see that it is a necessary truth .

Our problem therefore becomes the following: is the theory of evolution a pack of vague generalities, or does it have an unambiguous meaning? Is it a modern theory or an old one? The best way to find out, I believe, is by looking at some ideas of the Classics. That will make us realize not only now modern and original the theory is, but also how precise its concepts really are.

"THE CHILDHOOD OF THE WORLD"

Multitudinous atoms, swept along in multitudinous courses through infinite time by mutual clashes and their own weight came together in every possible way and realized everything that could be formed by their combinations. So it came about that a voyage of immense duration, in which they had experienced every variety of movement and conjunctions at last brought together by sudden encounters those which form the starting point of the substantial fabrics-- earth and sea and sky and the races of living creatures.

The newly formed earth is seen as a cosmic female which was pregnant with all living organisms and Lucretius describes 'the childhood of the world' in this way.

There was a great superfluity of heat and moisture in the soil and wombs grew up wherever a suitable spot occurred. clinging to the earth by the roots. These, When the time was ripe were burst open by the maturation of the embryos which fled from the moisture and struggled for air. Then nature directed towards those spots the pores of the earth, making it open its veins and exude a juice resembling milk, just as nowadays every female when she has given birth is filled with sweet milk because all the flow of nourishment within her is directed into the breasts. The young were fed by the earth, clothed by the warmth and bedded by the herbage. Here then is further proof that the name of mother has rightly been bestowed on the earth.

Then because there must be an end to such parturition, the earth ceased to bear, like a woman worn out with age. For the nature of the world as a whole is altered by age, and everything must pass through successive phases. Nothing remains for ever what it was.

In those days the earth attempted also to produce a host of monsters, grotesque in build and aspect-- hermaphrodites, halfway between the sexes yet cut off from either, creatures bereft of feet or dispossessed of hands, dumb, mouthless brutes, or eyeless and blind, or disabled by the adhesion of their limbs to the trunk. These and other such monstrous and misshapen births were created. But all in vein. Nature debarred them from increase. For it is evident that many contributory factors are essential to the reproduction of a species. First, it must have a food-supply. Then it must have some channel by which the procreative seeds can travel outward through the body when the limbs are relaxed. Then, in order that male and female may couple, they must have some means of interchanging their mutual delight.

The fact that there were abundant seeds of things in the earth is no indication that beasts could have been created of intermingled shapes. The growths that even now spring profusely from the soil--the varieties of herbs and cereals and lusty trees--cannot be produced in this composite fashion: each species develops according to its own kind, and they all guard their specific characters in obedience to the laws of` nature.

In those days, again, many species must have died out altogether and failed to reproduce their kind. Every species that you now see drawing the breath of life has been protected and preserved from the beginning of the world either by cunning or by prowess or by speed. The surly breed of lions, for instance, in their native ferocity have been preserved by prowess, the fox by cunning and the stag by flight.

Again, men noticed the orderly succession of celestial phenomena and the round of the seasons and were at a loss to account for them. So they took refuge in handing over everything to the gods and making everything dependent on their whim.

Poor humanity to saddle the gods with such responsibilities! What griefs they hatched then for themselves, what festering sores for us, what tears for our posterity! This is not piety, this oftrepeated show of bowing a veiled head before a graven image; this bustling to every altar; this kow-towing and prostration on the ground with palms outspread before the shrines of the gods, this deluging of altars with the blood of beasts; this heaping of vow on vow. True piety lies rather in the contemplation of the universe with a quiet mind.

THE IDEA OF DECLINE

For Lucretius the decline was primarily biological, because the earth had become sterile and could not produce new species, except for insignificant little creatures, while many of the original species had been lost for ever. The diversity and the quality of life on earth could only decrease. The Bible does not say much on the natural history that followed the Creation, but the report that the patriarchs lived for hundreds of years (Adam lived to be 930 years old) indicates that the idea of biological decline was known and accepted. The same idea is present in the legends of many primitive tribes, which invariably speak of a 'Golden Age' at the beginning, of better, healthier and heroic ancestors

From a religious point of view, the idea of decline makes sense because God is perfect and must have created a perfect world. But He also allowed freedom to exist, and therefore evil, and evil could only lead to a deterioration in the original word

What is less obvious is why the idea of decline was shared even by men like Leucippus, Democritus and Lucretius who professed a total materialism. Why did they think that life had to explode at the beginning in the full glory of its innumerable forms? Why did not they, or their followers, conceive the idea that only a few simple creatures appeared on earth, and slowly evolved into more diverse and more complex forms?

After all, the universe had originated, for them, from the chance encounters of atoms which arranged themselves in all possible combinations. There had been therefore an increase in complexity in the physical world, from the lowest to the highest possible levels. Why did they not extend this idea to the biological world?

There may be many good reasons for this, but one, I believe, is fundamental. They conceived the idea that the physical world is made of units, or atoms, but nobody thought that the biological world is also made of units, or cells. If they had thought of that, it would have been possible to imagine that the cells gave origin to increasingly more complex combinations as the atoms did, and the idea of biological evolution could have been born.

But they didn't. The classic philosophers conceived many ideas that later became science; they thought not only of atoms, energy and wave motion, but also of relativity and indeterminism. For some reason, however, the idea of the cell was unthinkable, and without it the concept of organic evolution could not germinate

Some people may think that this explanation is too trivial, but they should think again. We are far too accustomed to take the cell for granted, and are largely unaware that to think of life in terms of cells has been one of the greatest theoretical revolutions in the history of mankind.

At any rate, it is an historical fact that the idea of evolution from microorganisms to animals and plants started circulating after the discovery of the microscope and of microscopic life. That, in my opinion, is not a coincidence. Just as it is not a coincidence that our understanding of the cell and of evolution have gone hand in hand ever since.

A NEW BEGINNING

As we have seen, both the De Rerum Natura and the Bible spoke of a link between present and past organisms, the idea that life had a history and that changes occurred during this history was already there, and there is nothing new in it. The real difference between Evolution and all previous concepts of historical Change IS the idea of what happened in the beginning, of what creatures came into existence first.

If only a few types of organisms appeared on the primitive earth, then a great process of diversification must have taken place during the history of life. If the first creatures were invisible microorganisms, their descendants had not only to become different, but some of them had to develop into bigger and more complex forms of life.

Evolution is the idea of an historical change accompanied by increasing diversity and increasing complexity, and its very foundation, therefore, is the hypothesis of the ancestral microorganisms, the idea that life on earth began with one or a few types of cells.

Today we look at this hypothesis from a rather comfortable position. We have a fairly good idea of the age of the earth-- around 4.6 billion years--and from the fossil record we know that for almost three thousand million years--roughly from 3.6 to 6 billion years ago--the microorganisms were the sole inhabitants of our planet. They were not only the first creatures to appear, but had the earth to themselves for about two thirds of its history, and today they are still the predominant form of life (they account for at least 95% of all living matter).

All this information however is recent history. The first microfossils were discovered in 1954, and before that date there wasn't the slightest evidence for the idea of ancestral microorganisms How was it possible then to conceive such an idea in the l8th century? How was it that a few men of science came to think what no philosopher had ever thought before, what no poet had imagined and no experimental fact had documented? The story began, as so many others in science, with the invention of an instrument.

Antoon van Leeuwenhoek announced the discovery of the microscope in a letter to the Royal Society of London in 1676 and in 1683 sent another letter that described the first 'microscopic observations about animals'. His drawings, published in 1684, were the very first pictures of bacteria and protozoa. Leeuwenhoek was also the first person to observe spermatozoa under the microscope, and it was this report, more than anything else, which fired people's imagination and spread enthusiasm for the new science of microscopy.

Here at last one could see what is starting life: a tiny, wriggling, tadpole-like infinitesimal creature which had a round head and a tail. The excitement produced a chaotic stream of reports, and it is difficult for us to differentiate what people were seeing under the microscope from what they imagined they were seeing, but that is understandable. Scientists had to get used to the idea of microscopic life, had to convince themselves that microorganisms are not freaks, that they are everywhere and not just in some curious places, and that some progress could be made by building better microscopes.

Microbiology and Embryology grew very slowly, but they grew together, and inspired a diffused belief in the idea that there is a parallel between the general development of life and the individual development of the embryo. In 1744, Albrecht von Haller coined the word evolution to indicate the unfolding of an embryo, but later the word came to be used for the development of life as a whole. This linguistic transformation reveals only too clearly the emerging feeling of the time. If the beginning of every organism is a single cell, perhaps life on earth started in the same way, from invisible microscopic creatures.

Today the origin of the cell is an enormous puzzle for us, but in the 18th century it was no problem at all. The belief in spontaneous generation was genuine and widespread (despite the experiments of Redi and Spallanzani), and provided the perfect answer. All the necessary ingredients therefore came together. Microscopic life is full life, it can produce visible creatures, and can originate naturally and suddenly by spontaneous generation. The idea of evolution 'in principle' was born.

The fact that this was pure speculation should not deceive us. Since there was no evidence, scientists pushed the idea of the ancestral microorganisms into the background, and concentrated on the other problems of the history of life, in particular on the 'transformation' of species. But that 'speculation' was the very heart of the young theory, because there would be no point in talking about an increase in diversity and complexity if the original creatures had not been less diverse and less complex.

The microscope made it possible to see and think what nobody had seen and thought before, and it doesn't really matter if scientists did not talk or write openly about life beginning from invisible creatures. The important point is that for the first time in history they could think about it and find it credible. The formal publication of the new theory, at any rate, was not far away.

LAMARCK

1. Life appeared on earth in the form of microorganisms, by spontaneous generation.
2. Organisms diversified and adapted to the environment by the inheritance of acquired characteristics.
3. Organisms became more complex because there is an intrinsic tendency in every creature to reach higher levels of complexity.

Lamarck, on the other hand, saw in this obscure force the effective driving power which shaped the history of life. 'Evolution by Internal Drive' was the mechanism proposed by Lamarck as 'Evolution by Natural Selection' was the mechanism proposed by Darwin.

There is however another way of looking at Lamarck's contribution. He proposed the wrong mechanism, but we should keep in mind that the theory and the mechanism of evolution must be clearly separated. This can be done easily by eliminating the last part from Lamarck's hypotheses, which now read as follows:

1. Life appeared on earth in the form of microorganisms.
2. Organisms diversified and adapted to the environment.
3. Organisms became more complex.

There is another fact which proves this point. The visible expression of the theory of evolution is the 'Tree of Life', a diagram which represents different creatures linked together as if they were the members of the same genealogical tree. Lamarck was the first person to draw such a diagram. It was a very simple sketch (Figure 1), with only a few entries, but the essence of what we call phylogeny (the development of the species) is all there.

Again, the details are wrong (he proposed that whales and land mammals derived from seals, for example), but that is hardly surprising. Darwin did not even attempt to draw a real phylogenetic tree (the diagram that he put in the Origin of Species--reproduced in Figure 2--does not represent actual organisms), and the first complete phylogenetic tree, published by Ernst Haeckel in 1866--does not have fewer mistakes than Lamarck's ( Figure 3).

The first orang-utan that was brought to Paris caused a sensation, according to Diderot, and in 1770 De Lisle de Sales suggested that the orang-utan had been the ancestor of mane for which he was duly sent to prison. The idea that we descend from the apes was interesting or disturbing, according to one's point of view, and in any case dangerous, but it had not required a great leap of the imagination: a good look at an ape had been enough. It is one thing, however, to recognize familiar features in apes,

FIGURE 2: Darwin's illustration of the principle of divergence and the origin of species (l859)

quite another to say that we are related to birds, snakes, whales and many other animals around us. That is truly beyond all appearances.

It is odd that people have been unable to give Lamarck credit for this great vision, and remember him only for the inheritance of acquired characteristics. Darwin too proposed a theory of inheritance which is equally discredited (the theory of Pangenesis), but this is hardly mentioned. Why people chose to remember Darwin only for his good ideas and Lamarck only for his bad ones is a mystery to me.

At any rate, when Lamarck published the first phylogenetic tree, in 1809, he effectively wrote the 'manifesto' of evolution, the programme for an immense research project which is still going on The theory of evolution was born then and there. The great achievement of Darwin, 50 years later, was to find a convincing mechanism for it, and make it popular.

NATURAL SELECTION

When the Penicillium mold was first isolated, its natural production of penicillin was 30 units per milliliter, and in order to increase this yield the pharmaceutical companies embarked on one of the largest programmes of biological modification that has ever been undertaken. Today, the industrial descendants produce 30,000 units of penicillin per milliliter, and yet for all the variations and selections that it has gone through the mold is still a Penicillium, it has not changed species (Sermonti, 1981).

The same is true for the experiments where a preferential outcome was deliberately avoided, and biologists just watched what happened in cell cultures under different conditions. The appearance of new varieties of microorganisms from a single strain has been described in detail countless times, but not the appearance of new species.

Let us examine these experiments with cell cultures a little more closely, because they are an invaluable source of information: we can easily grow billions of cells in small dishes, and observe many generations of descendants in a relatively short time.

If we divide a heterogeneous population of cells in equal groups, for example, and cultivate them in dishes which provide different environments, some generations later we usually find that each dish has its own predominant form of microorganisms. We learn in this way that the strength or the weakness of each form is not an absolute quality, but something that depends very much upon the environment.

Let us now take groups of identical cells and grow them in conditions which are kept rigorously equal in all dishes, but which are not ideal for that type of cell. Some generations later, we usually find that in some dishes the cells have changed and grow better than in others. The important point here is that at the beginning of the experiment there was no way of deciding in which dish the changes would take place, when and if they would take place, and what types of change would take place.

We learn in this way that biological variations occur at random.

In these experiments we come face to face with two basic processes. We see that changes in hereditary characteristics take place at random, and we see that the environment has a critical role in determining which variety has more success in reproducing itself. As if the environment had an invisible hand which operates a natural process of selection.

Variation and selection are clearly different things, but biologists have been used to describing them as the two phases of one process, and have given to this two-step mechanism the name of "natural selection". This is not the clearest way of describing what happens, but the terminology doesn't really matter. The important point is that we can convince ourselves that natural selection not only exists, but really works: we can actually see it in action before our own eyes in the laboratory.

But we also see that only new varieties appear, not new species, as if species had the ability to bend but not to break when the environment changes. Natural selection, in other words, has all the characteristics of a mechanism that works for the conservation of existing species, not for the creation of new ones.

This conclusion is by no means valid only for microorganisms. Centuries of crossing and breeding experiments with animals, crops and plants, sometimes performed on massive scales, have shown the same pattern time and time again. Many new varieties have been obtained, but no human being has ever seen and recorded the formation of a new species in Nature.

DARWIN

As we know, 21 years later Charles Darwin and Alfred Russel Wallace examined the same question in their memoirs to the Linnean Society (read together on July 1st, 1858), and answered in the affirmative: natural selection is the mechanism by which new species are formed during the history of life.

Did Darwin and Wallace have new conclusive evidence? No, they didn't have a single case in which the origin of a new species had been seen and documented. Did they show that Blith's reasoning was wrong? Again, no. The conclusion that species conserve themselves by adapting to the environment was (and still is) based on too many facts to be challenged. How then did they manage to turn the role of natural selection upside down, and convince quite a number of biologists that that most conservative force is in fact the creative power of evolution?

Darwin explained this in detail in the Origin of Species (1859). What the evidence shows, he argued, is that variations do occur in the hereditary characters, and that they occur at random. Furthermore, the evidence shows that variations usually come in small jumps, because monstrosities or sports as a rule are not viable. There is therefore in Nature what Wallace called the Tendency of Varieties to depart indefinitely from the Original Type, and which Darwin described as the Principle of Divergence.

Let us now attribute this natural property of divergence to the most primitive creatures, and follow with the eyes of the mind what happened. The descendants of the ancestral creatures gave origin to new varieties, the descendants of each variety diversified further, and the descendants of the descendants went on diverging in countless varieties, like a tree that started from one seed and keeps producing more and more branches.

But not all varieties could survive. Many died, and left a complicated series of gaps in the network. In this way, the branches of the Tree of Life formed irregular patterns, like those of a real tree, and arranged themselves in major, medium, minor and minute groups to which we give such different names as phyla, orders, genera and species.

When we look at different species we may think that a creative force had to be responsible for the diversity, while in fact it is only because their intermediate forms disappeared that we perceive them as separate groups. The discontinuity between species is real only because the gaps were produced by real extinctions, not because there was a creative process of speciation which differs in principle from the standard process that gives origin to the varieties. Ultimately there are only varieties in Nature, according to Darwin:

Varieties, then, have the same general character as species, for they cannot be distinguished from species . . . except . .. by a certain amount of difference, for two forms, if differing very little are generally ranked as varieties, notwithstanding that intermediate linking forms have not been discovered, but the amount of difference considered necessary to give to two forms the rank of species is quite indefinite.

Lamarck, as we have seen, had already spoken of links between different species, but he had also believed in spontaneous generation, and therefore in the possibility that different microorganisms appeared at different times and became the progenitors of separate trees of life. In Darwin's framework, spontaneous generation has no place, and the unity of life therefore runs all the way through the organic world, embracing all creatures of the present and linking them to all creatures of the past.

We are here today because there came before us "a finely graduated organic chain", a procession of "a truly enormous number of extinct varieties" which have slowly built all the steps of all the routes that link every creature to the original ancestor.

Darwin summarized this vision in the final words of his book:

There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one, and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

CUVIER

Cuvier held the chair of Anatomy at the Museum National d'Histoire Naturelle in Paris (the Institute where Lamarck was working), and it was with precise anatomical arguments--he was the founder of Comparative Anatomy--that he gave substance, method and credibility to the study of fossils.

The first problem we face in Paleontology is that, in most cases, all we have of past organisms is a few bones, and these would tell us very little if it were not for the first golden rule of Cuvier, the Principle of the Correlation of Parts. This principle tells us that we can reconstruct a whole body from a small portion of it because every part is correlated with the others and in general is compatible with only one anatomical architecture. Cuvier himself made the drawings of entire organisms from a few bones, and when complete skeletons were discovered, after his death, people were amazed to find how close he had come to the real structures, even in small details.

The reconstruction of a whole body from a few remains, however, is only the first step. After that we have to place the reconstruction in its historical context, and here too we follow Cuvier. Let us illustrate this with an example. There were at the Museum bones of elephant-like creatures which had been found in Siberia, and in 1798 Cuvier was able to demonstrate that they had belonged to a species which differed from all existing elephants, the extinct species of mammoths.

It was the very first demonstration that species which once lived on earth had become extinct. More important, the demonstration was based on anatomical principles which are valid in general, and not just for that particular case, and in this way Cuvier gave us the second golden rule of Paleontology. Once we have reconstructed the structure of a past organism, we establish with Comparative Anatomy its relationship with known forms and decide if the creature in question has become extinct or not.

In addition to this theoretical work, Cuvier went in person to excavate around Paris, and made another fundamental discovery. He found that the fossils in the chalk caves of Montmartre. were distinctly different from those of the strata immediately above and below the caves. One layer had fossils of creatures which lived in the sea, the second of fresh-water organisms, and the third of animals which lived on dry land.

A succession of geological layers was revealing a succession of periods in the history of life, as if the layers were the pages of a giant book. It became possible to establish which creatures came before which, and from the thickness of the layers one could estimate, approximately, how long each period had lasted in respect to the others. Cuvier had discovered Stratigraphy, the most important tool of the paleontologists.
As for the theoretical implications, Cuvier was well aware of the ideas of Lamarck and his fellow 'transformists', of the suggestion that species are not immutable, that organisms had undergone gradual and progressive changes during the history of life, but he did not believe them. It may seem strange that the man who created Comparative Anatomy, Paleontology and Stratigraphy could not accept the idea of a gradual evolution of life, but he had two reasons for this.
In the first place, he firmly believed in the supreme priority of the facts, and to him the facts of the fossil record were saying that species had not changed gradually. They had existed unmodified for long periods of time, and then disappeared suddenly from the record, only to be replaced with equal suddenness by new species. The geological periods succeeded one another abruptly -- one could see this clearly written in the rocks--and the same was true for their forms of life: there was no trace of intermediate organisms in the record.
Cuvier candidly confessed that to him the sudden appearance of new species was the greatest mystery of all, but the records said that that is what happened, and to him that was that. The history of life had been a history of catastrophic extinctions followed by explosions of new forms, and no speculation could change that hard fact of Nature.
The second reason for rejecting evolution came from his discovery of a new biological category that he called 'embranchement', and that today we call 'phylum'. He compared various species of animals and discovered that the immense variety of body-structures that we find in Nature can be reduced to a few basic anatomical designs.
In the wings of birds and bats, for example, we find the same types of bones that exist in our arms, in the flippers of seals and in the forelimbs of reptiles and amphibians. The size and the shape of each bone varies enormously from one species to another, and yet we can clearly recognize which is which simply by its position and its relationship to the other bones. We say that the forelimbs of all vertebrates are homologous, because they have the same structural design, as if they had been obtained by modifying a common prototype-limb.

The wings of insects, on the other hand, have no bones. They are analogous to the wings of birds because they are used for the same function, but their structural plan is totally different. The important point is that a structural difference in a single part means that the anatomical plan of the whole body is different.

All animals which have a skeleton inside the body, for example, have the nervous system in a dorsal position and the heart in a ventral position. Those which have an outside skeleton (insects, crustacea and spiders) have that order reversed: the nervous system is ventral and the heart is dorsal.

All organisms that have the same basic anatomical design form one group--a phylum--, and Cuvier proved that there are at least 4 different phyla in Nature. Today most biologists recognize at least 25 phyla, and it is universally accepted that the phylum is a biological category as fundamental as the species. The anatomical barrier between different phyla is as real and as natural as the reproductive barrier between different species.

The existence of separate phyla was for Cuvier incompatible with evolution. In principle he could concede (if it wasn't for the evidence of the fossil record) that a prototype-limb had the potential to become a hand, a flipper or a wing (the homologies are only too obvious), but a common origin for the wings of insects and birds, for example, was anatomically absurd.

Transformation within a phylum are conceivable, but transformations from one phylum to another are impossible, and since the phyla are irreducible, concluded Cuvier, they cannot have a common origin.

In an historic public debate with Geoffroy St. Hilaire, on February 15th 1830, Cuvier formulated his two great objections to evolution: the discontinuity of the fossil record, and the discontinuity of the biological phyla. They were directed against the theory of Lamarck (who died in 1829, blind and destitute, at the age of 85), but since Darwin's theory is equally based on gradualism and continuity, these objections have been the two most powerful arguments of the antievolutionists ever since.

BOREDOM AND TERROR

To Murchison, the discovery meant that he was witnessing nothing less than Creation itself: the Cambrian had to be the period chosen by God to populate the planet. Today, the precambrian rocks have been examined under the microscope and the fossil remains of microorganisms have been found in many of them, but Murchison's discovery has not lost its impact. The gap between Precambrian forms and animals already differentiated into various phyla still represents a jump of immense proportions.

The Cambrian explosion is the most remarkable discontinuity in the record, but at least four other major upheavals took place afterwards, at the end of the Devonian, Permian, Triassic and Cretaceous periods. They consisted of mass extinctions followed by the rapid expansion of new forms of life.

The worse catastrophe took place at the end of the Permian when, according to the catalogue of David Raup (1979) up to 96% of all species were destroyed "leaving as few as two thousand forms to propagate life" (Gould, 1981). The catastrophe was so indiscriminate that, according to Stephen Jay Gould, it would be more appropriate to talk of the "survival of the luckiest" instead of the "survival of the fittest" for the organisms that remained alive.

The most popular catastrophe, the one which wiped out the dinosaurs at the end of the Cretaceous, also cut across many barriers and hit indiscriminately: only small animals, weighing no more than 20 pounds survived (Valentine, 1978).

The evidence of almost two hundred years of Paleontology leaves no doubt that the pattern of abrupt change described by Cuvier is substantially correct, and is valid not only for a few great episodes, but represents the rule, rather than the exception, throughout the history of life.

Instead of finding the gradual unfolding of life, what geologists actually find is a highly uneven or jerky record; that is, species appear in the sequence very suddenly, show little or no change during their existence in the record, then abruptly go out of the record.

The theory of Punctuated Equilibria, proposed by Niles Eldredge and Stephen Jay Gould in 1972, describes the same scenario: the history of life has been a sequence of "punctuations" or rapid changes, followed by "equilibria" or long periods of stability. "It was--wrote Derek Ager--like the life of a soldier ... long periods of boredom and short periods of terror".

There is however a fundamental difference between our perception of the past and that of Cuvier: radioactivity has allowed us to measure the absolute age of the rocks, and has enormously extended the temporal dimension of geology. Now we know, for example, that the peak of the Cambrian explosion (some 600 million years ago) lasted approximately 50 million years, while its tails covered between 150 and 200 million years. The dinosaurs disappeared some 65 million years ago at the end of a process that lasted at least 10 million years, and the same amount of time elapsed in the Great Catastrophe of the Permian, around 225 million years ago.

An interval of 10 million years is less than 0.3% of the age of the earth and in geological terms is short, but in biological terms -- in terms of numbers of generations--it is a very substantial length of time. What was sudden for Cuvier is still geologically sudden for us, except that instead of centuries we are now talking of millions of years, and this does make a difference. An abrupt transition in the fossil record may look as if the change took place from one generation to the next, while in fact there was time for hundreds of thousands of generations even in the fastest transitions on record.

This leaves the door open to the possibility that new forms of life derived from previous forms in rapid bursts of evolutionary activity, and recently the fossil record has revealed a pattern which is consistent with this interpretation.

MICROEVOLUTION AND MACROEVOLUTION

The diagram of Sepkoski and other similar models (Stanley, 1975 and 1979; Gould, Raup, Sepkoski, Schops and Simberloff, 1977; Gould, 1978) gave new life to the old analogy that a species is not just a collection of individuals but a super-individual, a natural organism made of distinct creatures as any creature is made of distinct cells. According to the analogy, species are born, live and die as individuals do, for example as the microorganisms of a cell culture. The analogue of a cell is a species; the analogue of cell division is speciation; the analogue of cell death is the extinction of a whole species.

One may point out that the analogy requires a very high number of past species to be valid, but this is not an obstacle. At least 99.9% of the species which appeared since the Cambrian explosion have become extinct, and since there are today at least 4 million species on earth, this puts the total number of past species at about 4 thousand millions, at a conservative estimate.

If the analogy is valid, the Cambrian explosion was the rapid phase of growth of a colony of species, no more mysterious than the rapid phase of growth of a colony of cells, and the phenomenon therefore becomes a perfectly natural event. The analogy can then be taken a step further. If we damage a cell culture, or subtract food, the cells start dying and the colony faces extinction; if we restore viable conditions in time, the colony expands again. In this way we observe a pattern of extinctions, expansions and long periods of stability that gives us a model not only for the Cambrian explosion, but also for what happened throughout the history of life.

But there is a price to pay: the analogy stands only if we admit that speciation is as real as cell division is, and at present we know nothing about the genetics or the biochemistry of speciation. New species may have originated by the accumulation of many small jumps--as Darwin suggested--or by a totally different mechanism: we simply do not know.

Models for an alternative mechanism of speciation have been advocated ever since Darwin's times, and are still actively debated. In 1940, for example, Richard Goldschmidt gave the name of micromutations to the genetic changes that produce new varieties within a species, and macromutations to those that produce new species altogether (by "a repatterning of the chromosomes"). There are two distinct types of genetic change, according to Goldschmidt, and therefore two distinct types of historical change which he called microevolution and macroevolution.

Microevolution does not lead beyond the confines of the species, and the typical products of microevolution, the geographic races, are not incipient species. There is no such category as incipient species. Species and the higher categories originate in single macroevolutionary steps as completely new genetic systems (Goldschmidt, 1940).

The reductionist view that evolution can ultimately be understood in terms of genetics and molecular biology is clearly in error. We must turn not to population genetic studies of established species, but to studies of speciation and extinction in order to decipher the higher-level process that governs the general course of evolution (Steven Stanley, 19755.

What was a conclusion for Darwin, is therefore a starting point for us. The beginning of a process that hopefully will reconcile paleontology with evolution, or Cuvier with Lamarck and Darwin, after more than a century of bitter controversies.

TIMETABLES AND PRIORITIES

In the 19th century, the period that came before was regarded as a mere preliminary stage, and was simply called Precambrian; now we know that that period covered 87% of the history of the earth. The feeling that we have been studying only the last episode of a very long story, that perhaps we should rearrange our approach and our priorities, is only too natural in these circumstances.

There are some, however, who resist this change on the grounds that time is not everything. It is possible, for example, that during the 3 billion years of cellular history life developed extremely slowly. Nobody denies that that period was important, but probably it was not as important as its enormous duration seems to suggest. Perhaps things really started to happen fast only with the Cambrian explosion, and what came before was only an immensely slow and sluggish preparatory stage.

This point of view has some justification if one looks at appearances: even in the Cambrian, for example, life was very primitive indeed. No animal was walking on the land, no insect nor bird was flying, no grass, no plants and no flowers were decorating the landscapes, and probably no fish were swimming in the seas.

And yet, crawling at the bottom of the oceans, there were strange-looking creatures which had already developed the characteristics of all major animal phyla. We must look at them "with speculative eyes", said George Gaylord Simpson, otherwise we understand nothing because external appearances are to a large extent irrelevant in Biology.

Simpson summarized the history of the Phanerozoic with the scheme of Figure 4, which shows that no new major phylum has been invested since the Cambrian explosion. Furthermore,

in spite of possible exceptions involved in the largely verbal question of defining 'phylum', it remains true that no major basic type of animal organisation is known ever to have become extinct (Simpson, 1949)

The invention of countless variations from a few designs has changed the face of the earth many times over but cannot be compared, in quality, with the invention of the basic designs. As Simpson put it:

The Cambrian age of life already represents an increase which throws into shade the increase from Cambrian to Recent.

Three centuries ago, the microscope revealed a new world of life, and the story of evolution began when people started imagining ancestral microorganisms. Since 1954, the ever-surprising microscope has shown that that imagined world is an historical reality--a three billion years long reality--and evolution has taken a dramatic new turn: we are going back to the microorganisms. After so much wandering, we are at last beginning at the beginning.