Darwin’s great book, “The Origin of Species,” comprised two quite distinct elements. In the first place, it demonstrated, with a vast wealth of examples, that the current theory of the fixity of species was untenable, whether in its theological guise of special creation or in any other form; it simply would not fit the facts of nature. In the second place, Darwin proposed a mechanism to account for evolution—the theory of Natural Selection, by which favourable varieties would automatically be accumulated and the apparent purposefulness of life could be accounted for in straightforward mechanistic terms.
It was this latter element which gave Darwin’s work its influence among professional biologists. Many of them were ripe for conversion to the idea of evolution, but before 1859 no one had put forward any but the most improbable suggestions as to how evolution could have been brought about. T. H. Huxley, for instance, records how, when he read the “Origin,” he said to himself, “How stupid of me not to have thought of that” and from then on became the champion of Darwinism.
This Darwinian view of evolution was generally accepted by biologists in the latter part of the last century. But about 1890 doubts began to be thrown upon it, and around 1910 it had become so unfashionable that some critics proclaimed the death of Darwinism. By this, of course, was meant the selectionist theory of the method of evolution: the fact that evolution has occurred was never seriously questioned after 1859.
This sceptical attitude was due to two main causes. For one thing, orthodox Darwinism was tending to become purely speculative, invoking natural selection to explain anything and everything without requiring proof and without providing any explanation of the machinery by which the results could be brought about. For another, genetics had discovered the fact of mutation—in other words, that hereditary change proceeds by jumps; and the theory was advanced that evolution, too, proceeded by large jumps, not by the gradual change which was the keystone of Darwin’s view.
In the last twenty-five years, however, many new facts about evolution and heredity have been discovered, and the balance has now swung over heavily and, I think, permanently, in favour of Darwinism. Chief among these new facts is the discovery that most mutations are not large but very small steps of change.
The net result of the last quarter century’s work in biology has been the re-establishment of natural selection as the essential method of evolution and its re-establishment not merely where Darwin left it, but on a far more secure footing. For one thing, the alternative explanations have ceased to be plausible. First among these is Lamarckism, or the so-called inheritance of acquired characters (which means the inheritance of characters acquired by an individual as a result of changes in the environment, like tanning due to sun, or of use or disuse of organs, like the more powerful muscles of the athlete or heavy worker; it does not refer to characters “acquired” through new mutation). This has now been thoroughly discredited.
Second, there is orthogenesis, or evolution in a predetermined direction, supposedly due to the germ-plasm being predestined to vary only in a certain way. It is true that when we can trace the actual course of evolution by means o^ abundant fossils, we often find that it does proceed in straight lines—the most familiar example is the steady evolution of the horses towards speed and the one-toed foot and towards elaborate teeth for grinding grass—but wherever (as is in most cases obvious) the direction is towards greater efficiency, this is to be expected on the basis of natural selection. In any case, there are some examples, like that of the elephants or the baboons, where evolution is not in a straight line, but changes direction during its course. There are a few puzzling cases, like the trend towards apparently useless or harmful characters, as seen in a number of groups of Ammonites shortly before their final extinction; but these are quite exceptional, and may prove to be susceptible of alternative explanation. In any case, orthogenesis in a useless or harmful direction would demand mutation-rates much higher than any yet found in nature.
There are also the vitalistic theories of a mysterious life-force or unconscious purpose, like Bergson’s elan vital. However, these are in reality not explanations at all, but mere confessions of ignorance. To say that life evolves because of an elan vital is on a par with saying that a locomotive runs because of an elan locomotif.
Not only have the alternative explanations become implausible, but a great deal of new support has been forthcoming for the theory of natural selection. One of Darwin’s difficulties about his own theory (which caused him to give greater weight to Lamarckism than he would otherwise have done) was that he could not see how new hereditary variations of small extent—what we to-day should call small mutations—could be preserved and kept from being swamped by crossing. This, as R. A. Fisher has pointed out, was due to his acceptance of the idea, current in his time, of “blending inheritance.” In a cross between two distinct types, whatever constituted the material basis of heredity (and Darwin’s generation completely lacked concrete knowledge on this subject) was supposed to blend in the resultant off-spring, as two drops of coloured ink will blend with each other. Thus, any new character would be quite literally diluted on crossing with the original type, and would soon fade out. The essence of Mendelism, however, is that the genes or units of heredity remain unchanged (apart from rare mutation) however they are combined with other genes. Many of the new genes produced by mutation can remain in the stock indefinitely until conditions are favourable, when they will begin to increase their representation in the stock. If a new mutant gene is recessive—i.e., must appear in double dose before it produces any visible effect—it can be carried in single dose for an indefinite period, even if it is slightly deleterious.
What is more, we now know that the effects of genes can be markedly altered by other genes, and numerous examples exist where slightly deleterious genes have been rendered harmless or even beneficial by being “buffered,” in the chemist’s phraseology, by new combinations of other genes. A beautiful example comes from domestic dogs. In producing the show type of St. Bernard, man has encouraged features characteristic of abnormal overgrowth of the pituitary gland: yet St. Bernards are not themselves abnormal, as a man with comparable characters would be. However, when St. Bernards are crossed with other breeds like Great Danes, a considerable number of the offspring show actual pathological symptoms. In producing his ideal of a St. Bernard, man has selected for genes making the pituitary abnormal: but he has also aimed at healthy dogs and so has automatically selected for other genes which would prevent the first genes from exerting any major harmful effect. But when these “buffering” genes are diluted or reduced in number by crossing, the potential abnormality of the pituitary can become actual.
This fact of recombination is the source of a whole category of variation unsuspected by Darwin: much that is new in evolution is due, not to wholly new genes produced by mutation, but only to new combinations of old genes.
Still another fraction of the raw material of evolution depends on the fact that the genes are arranged in a row along a series of visible, (but of course microscopic) threadlike bodies called the chromosomes. Owing to accidents in cell reproduction, whole sets of chromosomes may be added or subtracted. Doubling of the normal complement of chromosomes is a frequent subsidiary method of evolution in plants. The polyploids, as the types with increased chromosome-number are called, are often more resistant to extreme conditions: for instance, polyploids constitute an unusually large proportion of the varieties found in the arctic and mountain regions that have become re-colonized since the retreat of the ice after the Ice Age.
Chromosome-doubling may also occur after a cross between two true species. In this case, a new species is formed at one jump—a process which would have shocked most of Darwin’s nineteenth-century followers, who believed that all evolution was gradual. Sometimes such new types are weakly, and die out; in other cases the new combination of genes gives them exceptional vigour, and they may even oust both their parents. The classical example of this comes from the rice-grasses, Spartina, which live on mud-flats. During the last half-century, a new type of rice-grass appeared in Western Europe, and has been so successful that the Dutch have used it to reclaim land from the sea. Investigation has proved that this is a new polyploid species produced by the crossing of an original European species with one accidentally imported from America. In some areas the European species has been virtually exterminated by the new type.
Single chromosomes or groups of them may also be added or subtracted to give favourable results: a cytological accident of this sort gave rise, it seems, to the very successful branch of the rose family which later produced the apples and pears and their relatives.
Finally, bits of chromosomes may be shifted about. Small sections may be repeated, thus increasing the total number of genes available. Sections may be inverted, which tend to isolate the genes they contain from those contained in the uninverted section. Or chromosomes may exchange sections, a process which will help in the reproductive isolation of the new strain.
Evolution does go by jumps, but in most cases the jumps are so small that they hardly ever take the new type outside the range of variation already existing in the species, and the visible result is a gradual one. Discontinuity of variation is thus translated into continuity of evolutionary change: instead of a staircase, life marches up a ramp.
So much for the mechanism of evolution. But Darwin was almost equally unprovided with knowledge about the actual course pursued by evolution in different groups and in different conditions. He was aware of the fact that fossils from an earlier epoch differed from the modern inhabitants of the region, though resembling them in general type; he was aware that isolation might play a role in the production of new species; he knew of animal or plant groups which were on the border-line between mere variety and obviously good species; he worked out for himself some of the results to be expected of sexual selection or competition for mates between rival males. But that, with the indirect evidence provided by comparative anatomy and geographical distribution, was about all.
With this meagre body of knowledge at his disposal, his genius was able to put evolution on the map; but he could not proceed to the further task of mapping evolution itself. That was reserved for the slow cumulative work of several later generations of biologists.
It is not easy to sum up the chief results of that later work in brief and intelligible form; but it must be attempted. First, there is the formation of new species. These originate in many different ways, and even those with the same origin may come to differ in size and internal structure. The chief method of origin is through physical isolation. Once two groups are physically isolated so that they can no longer interbreed, they inevitably come to diverge from each other in the new mutations and the new gene-recombinations which they accumulate under the influence of natural selection. And after a certain time the differences in their constitution reach such a pitch that, even if the two stocks are brought together once more, they are partially or wholly infertile on crossing.
In addition, when an isolated group is small in numbers, it can be shown on mathematical grounds that it is likely to pick up and incorporate some mutations and recombinations that are useless or even slightly unfavourable. Thus, some of the diversity of life is, biologically speaking, purely accidental.
The effects both of physical isolation and of small populations are well illustrated by the plants and animals of islands. A population on an island is more or less completely isolated from other groups: and, accordingly, islands have a disproportionate number of distinctive sub-species and species, different from the species inhabiting the nearest mainland and from those inhabiting other nearby islands.
The extraordinary number of distinctive species of giant-tortoises and ground-finches on the Galapagos Archipelago was one of the main facts met with by Darwin in his voyage on H.M.S. Beagle which convinced him of the reality of evolution. Again, there is only one form of mouse-deer on the whole of Sumatra and Borneo, while the Rhio-Linga Archipelago close by, with only one one-hundred and fiftieth of the area, boasts no less than seven distinct sub-species.
In the Adriatic a large number of islands have been formed by subsidence of the land since the end of the Ice Age. Many of them are inhabited by distinctive races of lizards. A recent study has shown that the smaller the island, and therefore the smaller its lizard population, the more different this has become from the mainland type from which it was originally derived.
The other chief method by which new species are formed is through genetic isolation. This happens when a new form, wholly or partly infertile when crossed with its parent, is produced by some genetic accident—by means of the reduplication of whole chromosome sets, with or without previous species-hybridization; by means of the subtraction or addition of whole chromosomes; or, in some cases, by the break’ age of chromosomes and the re-union of the pieces in new arrangements.
The result is an overwhelming multiplicity of distinct species. Naturally they are all adapted to their surroundings: but the geographical and cytological accidents that produced physical and genetic isolation cause their number to be much greater than that which would be necessary on purely adaptive grounds; and non-adaptive variation adds its quota to the diversity.
Most of evolution is thus what we may call short-term diversification. But this kaleidoscopic change is shot through with a certain proportion of long-term diversification in the shape of the long-range trends revealed in fossils by the palaeontologist and deduced from comparative studies by the morphologist. These trends may continue for a very long time—up to tens of millions of years; but they always come at last to a dead end. After this, minor diversification may continue at the species level, but no further improvement takes place in the major specialization. Thus, birds ceased to show any improvement as flying mechanisms some fifteen million years ago, and there has been no evolutionary improvement of the ant type for perhaps twenty-five or thirty million years.
Such trends in a given direction are to be expected on Darwinian principles. Improvement of teeth and claws for a carnivorous existence, for instance, will be an advantage to a small generalized mammal when there are no specialized carnivorous competitors already in the field, and will be favoured by natural selection. Once the type has become at all adapted to flesh-eating, it will be almost impossible for it to switch over to a herbivorous existence, for example; the number of mutations needed is much too great, and meanwhile any single mutation making for greater efficiency as a carnivore will be caught in the net of natural selection and incorporated in the constitution of the stock. The stock thus finds itself at the bottom of an evolutionary groove of specialization. Natural selection forces it further along in the same direction, while constantly deepening the groove and so making it ever more impossible for the stock to escape out of it into some other way of life. The dead end comes when the specialization is so near its maximum possible perfection that selection cannot force the stock any further.
A third and still rarer type of change is evolutionary progress, which escapes the dead end awaiting specialization. It does so because its essence is all-round improvement as opposed to the one-sided improvement that characterizes all specialization. It raises the general level of life’s performance, instead of merely improving performance in respect of one particular mode of existence.
The net result of evolutionary progress can be defined as the raising of the upper level attained by life in respect of certain very general properties: greater control; greater independence; greater harmony of construction; greater capacity for knowledge (and, we may probably add, for emotion) . More concretely, it has permitted the rise of a succession of what the biologist calls dominant groups, because they spread and evolve rapidly, cause the extinction of many representatives of other groups, and play a new and predominant role on the evolutionary stage. The last three dominant groups in life’s history have been the reptiles, the mammals, and man, each later one arising from an unspecialized branch of the one before. Most (or, in some cases, all) of the branches of a dominant group undergo specialization, and then eventually come to a dead end, either by ceasing to evolve, or by the still deader end of complete extinction, as with most of the reptilian specializations, like the Dinosaurs, Ichthyosaurs, and Pterodactyls.
I said that progressive lines were rare. If we define progress strictly as capacity for unlimited further avoidance of dead ends, there has only been one progressive line in the whole of evolution—that which has led in its later stages through fish, amphibian, reptile, and mammal to man; for it appears well established that all other lines have come to an evolutionary dead end well before the later part of the tertiary period.
Thus, in the broad view, evolution as a process consists of one line of unlimited progress among thousands of long-range trends towards specialization, each of these latter in turn beset with a frill, so to speak, of thousands of short-range diversifications producing separate species. Some of the peculiarities of these separate species are due to non-selective accidents; but all the rest have been closely guided and moulded by natural selection.
Darwin introduced time into biology, and forced us to regard human history as the extension of a general process of change, operating by an automatic natural mechanism. Darwinism to-day has fully confirmed these general conclusions, but has, in addition, enabled us to distinguish between different types of change, and to link up human with biological history more fruitfully by introducing the idea of progress and the criterion of desirable or undesirable evolutionary direction.
In addition, the modern extension of Darwinism has enabled us to analyse the process of selection in a way that was impossible in Darwin’s day. In the first place, the intensity of selection may vary very considerably, and this will be reflected in its results.
In the Great Lakes of Africa, nature has conducted a demonstrative experiment, by permitting powerful predatory fish to reach some lakes, but not others. The little fish known as Cichlids exist in all the lakes. Where predators are present, as in Lake Albert, only four different Cichlid species have evolved since the Ice Age; but where predators are absent, as in Lake Victoria, there are over fifty Cichlid species, adapted to many new habitats and ways of life. Predator pressure has a restrictive effect on the diversification of prey.
The same sort of thing has happened in Australia, where the early or marsupial type of mammal was isolated before the more efficient placental type had been evolved. Accordingly, as everyone knows, the marsupials in Australia have produced dozens of types, such as kangaroos, Tasmanian wolf, flying phalanger, not found either living or fossil in any other part of the world. Elsewhere the pressure of more efficient competitors has prevented this efflorescence, and only a few generalized marsupials, such as the American opossum, have survived.
The Australian marsupials illustrate another point. The Australian area is much smaller and less varied than the great land masses of the northern hemisphere where the higher placentals evolved. There is less scope for variation, less need for extremes of efficiency, so that general selection-pressure never became so intense. As a result, the Australian marsupials were not pushed so hard or so far along their lines of specialization as were the placentals; they were not forced to such a pitch either of bio-mechanical efficiency or of intelligence; and they at once go down hill and are threatened with extinction when they have to compete with introduced placental types.
Even more interesting are the recent studies on qualitative differences in the results of different kinds of selection or, if you prefer, of selection operating in different circumstances. Thus a peculiarly acute competition takes place before birth among mammals which produce several young at a time. More eggs are always fertilized than can survive to birth; there is thus an intra-uterine selection, which puts a premium on quick and vigorous growth, for any laggard embryos will fail to get their fair share of the available nutriment and will die and be resorbed or aborted. As J. B. S. Haldane has pointed out, this pre-natal rapidity of growth will tend to continue after birth; and so the slow growth and prolonged infancy which makes human learning possible could never have been evolved except in a mammalian stock like that of the monkeys, where only one young is normally born at a time.
Haldane has also drawn attention to the interesting point that instinctive altruism, such as is shown by bees or ants, cannot be evolved except in social organisms where reproduction is confined to a limited caste and the altruistic types are sterile.
The most far-reaching conclusion deriving from modern analysis, however, is that the results of natural selection are not necessarily beneficial to the species, and may even be harmful. This apparent paradox is based on the fact that much of the struggle for existence is not directed against the forces of nature, nor against enemies, nor against competitors of other species, but against other members of the same species. Not only does the species as a whole have to struggle (in a metaphorical sense) to survive and reproduce, but so do the individuals within it. In a given species of butterfly, for instance, only a small proportion of the young caterpillars will survive into the butterfly stage. But among these, the decision as to which shall reproduce may depend, for instance, on whether one can escape detection by its enemies better than others. Accordingly protective resemblance, as for instance, of the famous Kallima to a dead leaf complete with imitation veins and mould spots, may be pushed to an incredibly high pitch, and yet have no effect on the survival of the species as a whole, which will be decided mainly by the capacity of the caterpillars to survive their much more numerous dangers.
This intra-specific competition is most obvious when rival males compete for mates and most acute when polygamy prevails and success in reproduction thus brings a multiple advantage. When this is so, the characters which bring success in mating may become so over-developed as to embarrass their possessors in the struggle for mere existence, as with the train of the peacock or the wings—almost useless for flight— of the Argus pheasant. Sexual selection here has benefited none but certain types of males as against others: its results for the species as a whole are harmful.
Another old objection to Darwinian explanations of evolution is the incredible complexity of the detailed adjustments needed to effect a change such as the lengthening of an animal’s neck. To take but one example, all the tendons tying the neck vertebrae together must be strengthened and their direction adjusted. How could random variation and selection account for this? We now know that the tissue of which tendons are made, like many other tissues of the body, has the faculty of responding to demands upon it both by excess growth and by changes in the direction of its fibres. Granted this one basic adaptation, all the rest follow. The myriad detailed adjustments are not determined by heredity and selection, but are built anew in each individual during its development.
In these and many other ways, our modern knowledge of growth and development has lightened the burden on natural selection, at the same time that advances in heredity have shown natural selection to be a much more flexible instrument than the last generation of biologists thought possible.
To sum up, Darwinism to-day is very much alive. In certain respects, indeed, modern evolutionary theory is more Darwinian than Darwin was himself. Darwin’s special contribution to the evolution problem was the theory of Natural Selection, but owing to the rudimentary state of knowledge in certain biological fields, he was forced to bolster this up with subsidiary Lamarckian hypotheses, of the inheritance of the effects of use and disuse and of modifications produced by the direct agency of the environment. To-day, we are able to reject these subsidiary hypotheses, and can demonstrate that Natural Selection is omnipresent and virtually the only guiding agency in evolution.
Darwin has with some justice been called the Newton of biology. Like Newton, he gave his science a unifying concept, and one capable of extension into every corner of its field. There are evolutionary implications in every branch of biology. The human physiologist may provide the most detailed physico-chemical analysis of some bodily process: but his description will be incomplete unless he takes account of its evolutionary history as well.
Evolution, too, was one of the first branches of enquiry to demand that relativist point of view which is becoming increasingly central to the modern scientific outlook. The single organism, looked at through evolutionary spectacles, has no meaning except in relation to a particular environment, to a particular set of enemies and competitors, to a particular past history, and to a particular set of potentialities for the future. All this was implicit in Darwin’s masterly formulation of the problem.
The implications for man and for his general conception of nature and of his own place in nature, are equally far-reaching. The idea of a past Golden Age vanished into smoke; so did all static conceptions of human life. In their place we see inevitable change and possible progress, while at the same time the time-span of the human drama is enlarged a thousand-fold in the past and still more in the future.
Newton showed that the same general principles applied to the motion of heavenly bodies as to that of the humblest terrestial objects. Similarly, Darwin with his few simple principles of the struggle for existence, natural selection, and consequent adaptation, linked man with all the rest of life, from monkeys and flowers to bacteria and amoebae, in a common web of necessity and change. The fundamental principles of Newtonian physics have now been superseded (though it still remains as the most effective first approximation to physical truth). Though Darwin’s principles have been more modified in detail than Newton’s, there seems less likelihood of their being superseded by a different set of basic principles. There are no signs that evolutionary biology will not indefinitely remain Darwinian.