Waddington has gone even further by maintaining that if natural selection works primarily in favour of plastic, adaptable behaviour, then the process of canalisation during development will become so flexible in itself that it no longer requires any particular gene mutation to endorse the new feature, merely 'some random mutation to take over the switching function of the original environmental stimulus. The type of hereditary change envisaged by Baldwin is, therefore, much more likely than he could have realised.' [11]
The point will perhaps become clearer if we draw a parallel between the role of chance in evolution and in scientific discovery. Behaviourists tend to ascribe any original idea to pure chance. But the history of science teaches that most discoveries were made by several people independently from each other, at more or less the same time;* and this fact alone (apart from all other considerations) is sufficient to show that when the time is ripe for a given type of invention or discovery, the favourable chance event which sparks it off is bound to occur sooner or later. 'Fortune fayours the prepared mind', wrote Pasteur, and we may add: fortunate mutations favour the prepared animal.
* See The Act of Creation, pp. 109 ff.
A stupid and industrious scholar could indeed write a history of science as a history of lucky hazards: Archimedes' overspilling bath-water, Galileo's swinging chandelier, Newton's apple, James Watt's tea-kettle, Harvey's fish-heart, Gutenberg's wine-press, Pasteur's spoilt culture, Fleming's nose-drip -- and so on and so forth, whether apocryphal or true does not matter. But he would have to be very stupid indeed not to realise that if that particular chance event had not occurred, a hundred others might have had the same triggering effect on the prepared mind -- -or on some other contemporary mind working in the same direction; and only a very perverse historian could fail to see that the primary cause and directing force of scientific progress is the curiosity and initiative of scientists, and not the random appearance of chandeliers, apples, tea-kettles and nose-drips 'in all and every direction'.
Yet it is precisely this perverse view which determines the orthodox interpretation not only of the evolution of new animal forms, but also of new patterns of animal behaviour. The only explanation that neo-Darwinian theory has to offer is that new forms of behaviour, too, arise from chance mutations affecting the nervous system, preserved by natural selection. If, apart from a few tentative studies, the evolution of behaviour (as distinct from the evolution of physical structures) is still an uncharted territory, the reason may perhaps be an unconscious reluctance to put the already strained theoretical framework of neo-Darwinian genetics to an additional test. To quote a very trivial example: an individual song-bird or jackdaw or sparrow, on spotting a predator, will give an alarm call, warning the whole flock. 'These alarm calls', Tinbergen points out, 'are a clear example of an activity which serves the group but endangers the individual.' [12] Are we really to assume that the 'wiring diagram' in the sparrow's nervous system, which releases the alarm call in response to a stimulus of predatory shape, arose by random mutation and was perpetuated by natural selection in spite of its negative survival value for the mutant? The same question could be asked concerning the phylogenetic origin of the ritualised mock-fights in a great variety of animals, including stags, iguanas, birds, dogs, fish. Dogs, for instance, sprawl on their backs as a token of defeat and surrender, exposing their vulnerable bellies and jugular veins to the victor's fangs. One is inclined to call this a rather risky attitude; and what is the individual survival value of not hitting (or biting, goring) below the belt?
One could add a whole volume of examples of complex, purposeful animal activities that defy any explanation by chance mutation and natural selection; and the list would actually have to start with a single-celled sea animal, a relative to the amoeba, which builds elaborate houses out of the needle-like spiculae of sponges. From this simple protozoan, without eyes or a nervous system, which is but a gelatinous mass of flowing protoplasm, through the architectural skill of spiders and insects, through bottle-raiding birds, tool-making chimpanzees and up to man, we find the same lesson repeated -- a display of patterns of instinctive and learned behaviour which cannot be explained by any twist of logic as the result of random changes in bodily structure. To quote Dr. Ewer: 'Behaviour will tend to be always a jump ahead of structure and so play a decisive role in the evolutionary process.' [13] In this light, evolution no longer appears as a tale told by an idiot, but rather as an epic recited by a stutterer -- at times haltingly and painfully, then precipitating in bursts.
Once More Darwin and Lamarck
There remains a hard core of phenomena that seems to defy explanation by any of the processes discussed so far, and to cry out for a Lamarckian explanation in terms of the inheritance of acquired characters. There is, for example, the hoary problem why the skin on the soles of our feet is so much thicker than elsewhere. If the thickening occurred after birth, as a result of stress, wear and tear, there would be no problem. But the skin of the sole is already thickened in the embryo which has never walked, bare-foot or otherwise. A similar, even more striking phenomenon are the callosities on the Africa warthog's wrists and forelegs, on which the animal leans while feeding; on the knees of camels; and, oddest of all, the two bulbous thickenings on the ostrich's undercarriage, one fore, one aft, on which that ungainly bird squats. All these callosities make their appearance, as the skin on our feet does, in the embryo. They are inherited characters. But is it conceivable that these callosities should have evolved by chance mutations just exactly where the animal needed them? Or must we assume that there is a causal, Lamarckian connection between the animal's needs and the mutation which provides them? Even Waddington, who does not completely rule out the possibility of Lamarckian inheritance, prefers to invoke the Baldwin effect and developmental canalisation though it is not easy to see how they can satisfactorily explain phenomena of this kind.
But on the other hand, it is equally difficult to see how an acquired callosity could conceivably produce changes in the gene-complex. Difficult, but not entirely impossible. It is true that the germ cells are set apart from other body cells in splendid isolation, but their isolation is not absolute: they are affected by radiation, heat and certain chemicals. It would indeed, as Waddington says, be 'unsafe to rule a priori out of court' the possibility that changes in the gene activities of body cells could, under certain circumstances, also cause changes in the gene activities of germ cells by means of hormones or enzymes. Herrick [14] has also kept an open mind on the problem. Waddington has actually produced a tentative model of directive mutation to indicate that at the present stage of biochemistry such a process is conceivable. [15]
It would not serve any useful purpose to rehash the arguments and counter-arguments, which have been repeated over and again. In a few years' time the whole battle might be of merely historical interest, like the Newton-Huyghens controversy on the corpuscular versus the wave theory of light. Darwinian selections operating on chance mutations doubtless occur, but they are not the whole picture, and probably not even a very important part of the picture for two simple reasons: first, because the range within which chance factors can operate is considerably narrowed down by the factors discussed before; and in the second place because in the present form of the orthodox theory the very term 'selection' has become ambiguous. It once meant 'survival of the fittest'; but, to quote Waddington for the last time: 'Survival does not, of course, mean the bodily endurance of a single individual, outliving Methuselah. It implies, in its present-day interpretation, perpetuation as a source for future generations. That individual "survives" best which leaves most offspring. Again, to speak of an animal as "fittest" does not necessarily imply that it is strongest or most healthy or would win a beauty competition. Essentially it denotes nothing more than leaving most offspring. The general principle of natural selection, in fact, merely amounts to the statement that the individuals which leave most offspring are those which leave most offspring. It is a tautology.' [16]
br /> The Lamarckians, on the other hand, have failed to provide experimental evidence for the inheritance of acquired characters which could not be interpreted -- or explained away -- on a Darwinian basis. That again proves nothing -- except that if Lamarckian inheritance occurs, it must be a rather rare event. It could not be otherwise, for if every experience of the ancestors left its hereditary trace on the offspring, the result would be a chaos of shapes and a bedlam of instincts. But some of the 'hard-core' cases make it appear at least probable that some well-defined structural adaptations, such as the thickening of the skin on our feet or the ostrich's callosities, which were acquired by generation after generation, did in the end lead to changes in the gene-complex which made them inheritable. Biochemistry does not exclude this possibility; and the almost fanatical insistence on its rejection is but one more example of the intolerance and dogmatism of scientific orthodoxies.
It seems, then, that neo-Darwinian and neo-Lamarckian modes of evolution are extreme cases at opposite ends of a wide spectrum of evolutionary phenomena. I have mentioned a number of these; but there is still one more to be discussed, which has a special significance to man.
XII
EVOLUTION CTD: UNDOING AND RE-DOING
Who has seen the wind? Neither you nor I. But when the trees bow down their heads, The wind is passing by. Christina Rossetti
There have been periods of 'adaptive radiation' -- sudden bursts of new forms branching out of the evolutionary tree in a relatively short time. Such was the reptilian outburst in the Mesozoic, or the mammalian outburst in the Paleocene -- the first about two hundred, the second about eighty, million years ago. The opposite phenomenon is the decline and extinction of evolutionary branches. It is estimated that for every one of the existing one million species, hundreds must have perished in the past. And, as far as one can judge, most of the lines which have not perished have become stagnant their evolution came to a standstill at various stages in the long distant past.
Blind Alleys
The principal cause of stagnation and extinction is overspecialisation. Take, for example, that charming and pathetic creature the koala bear, which specialises in feeding on the leaves of a particular variety of eucalyptus tree and on nothing else; and which, in lieu of fingers, has hook-like claws ideally suited for clinging to the bark of the tree -- and for nothing else. Its human equivalent -- minus the charm -- is the pedant, the slave of habit, whose thinking and behaviour move in rigid grooves. (Some of our departments of higher learning seem expressly designed for breeding koala bears.)
Some years ago, in the Yale Review, Sir Julian Huxley gave the following short summary of the evolutionary process:
The course followed by evolution appears to have been broadly as follows. From a generalised early type, various lines radiate out, exploiting the environment in various ways. Some of these comparatively soon reach a limit to their evolution, at least as regards major alteration. Thereafter they are limited to minor changes such as the formation of new genera and species. Others, on the other hand, are so constructed that they can continue their career, generating new types which are successful in the struggle for existence because of their greater control over the environment and their greater independence of it. Such changes are legitimately called 'progressive'. The new type repeats the process. It radiates out into a number of lines, each specialising in a particular direction. The great majority of these come up against dead ends and can advance no further: specialisation is one-sided progress, and after a longer or shorter time, reaches a biomechanical limit. . . . Sometimes all the branches of a given stock have come up against their limit, and then either have become extinct or have persisted without major change. This happened, for instance, to the echinoderms, which with their sea-urchins, star-fish, brittle-stars, sea-lilies, sea-cucumbers, and other types now extinct had pushed the life that was in them into a series of blind alleys: they have not advanced for perhaps a hundred million years, nor have they given rise to other major types. In other cases, all but one or two of the lines suffer this fate, while the rest repeat the process. All reptilian lines were blind alleys save two -- one which was transformed into the birds, and another which became the mammals. Of the bird stock, all lines came to a dead end; of the mammals, all but one -- the one which became man. [1]
But, having made this point, Huxley drew a conclusion which is much less convincing: 'Evolution', he concluded, 'is seen as an enormous number of blind alleys, with a very occasional path of progress. It is like a maze in which almost all turnings are wrong turnings.' [2]
This sounds just like the Behaviourist's view of the rat in the maze as a paradigm of human learning. In both cases the explicit or tacit assumption is once more that progress is governed by blind chance -- chance mutatiom preserved by natural selection, random tries preserved by reinforcement, and that is all there is to it.
Escape from Specialisation
In the three previous chapters I discussed a number of phenomena which reduce the factor of chance to a subordinate role. Now I propose to discuss one more line of escape from the maze, known to students of evolution under the ugly name of paedomorphosis, coined by Garstang nearly half a century ago. But although the existence of the phenomenon is recognised, there is little mention of it in the textbooks because -- like the Baldwin effect or the marsupial puzzle it runs against the Zeitgeist.* To put it simply, the phenomenon of paedomorphosis indicates that in certain circumstances evolution can retrace its steps, as it were, along the path which led to the dead end, and make a fresh start in a new, more promising direction. The crucial point here is the appearance of some useful evolutionary novelty in the larval or embryonic stage of the ancestor, a novelty that may disappear before the ancestor reaches the adult stage, but which reappear and is preserved in the adult stage of the descendant. The following example will make this involved process clearer.
* I am much indebted to Mr. D. Lang Stevenson for having called my attention to Garstang's work.
There is now strong evidence in favour of the theory, proposed by Garstang as far back as 1928, that the chordates -- and thus we, the vertebrates -- are descended from the larval stage of some primitive echinoderm, perhaps rather like the sea urchin or sea cucumber (echinoderm = 'prickly-skinned'). Now an adult sea cucumber would not be a very inspiring ancestor -- it is a sluggish creature which looks like an ill-stuffed sausage with leathery skin, lying on the sea bottom. But its free-floating larva is a much more promising proposition: unlike the adult sea cucumber, the larva has bilateral symmetry like a fish; it has a ciliary band -- a forerunner of the nervous system -- and some other sophisticated features not found in the adult animal. We must assume that the sedentary adult residing on the sea bottom had to rely on mobile larvae to spread the species far and wide in the ocean, as plants scatter their seeds in the wind; that the larvae, which had to fend for themselves, exposed to much stronger selective pressures than the adults, gradually became more fish-like; and that eventually they became sexually mature while still in the free-swimming, larval state -- thus giving rise to a new type of animal which never settled on the bottom at all, and altogether eliminated the senile, sedentary cucumber-stage from its life history.
This speeding up of sexual maturation relative to the development of the rest of the body -- or, to put it differendy, the gradual retardation of bodily development beyond the age of sexual maturation -- is a familiar evolutionary phenomenon, known as neoteny. Its result is that the animal begins to breed while still displaying larval or juvenile features; and it frequently happens that the fully adult stage is never reached -- it is dropped off the life cycle.
This tendency towards a 'prolonged childhood', with the corresponding squeezing out of the final adult stages, amounts to a rejuvenation and de-specialisation of the race -- an escape from the cul-de-sac in the evolutionary maze. As J.Z. Young wrote, adopting Garstang's views: 'The problem which remains is in fact not "how have vertebrates been formed from sea squirts?", but "
how have vertebrates eliminated the [adult] sea-squirt stage from their life history? It is wholly reasonable to consider that this has been accomplished by paedomorphosis.' [3]
Neoteny, in fact, amounts to a rewinding of the biological clock when evolution is in danger of running down and coming to a standstill. Gavin de Beer has compared the classical view of evolution (such as expressed in Huxley's image of the maze) to the classical view of the universe as a mechanical clockwork. 'On this view', he wrote, 'phylogeny would gradually slow down and become stationary. The race would not be able to evolve any further and would be in a condition to which the term "racial senescence" has been applied. It would be difficult to see how evolution was able to produce as much phylogenetic change in the animal kingdom as it has, and it would lead to the dismal conclusion that the evolutionary clock is running down. In fact, such a state of affairs would present a dilemma analogous to that which follows from the view that . . . the universe has been wound up once and that its store of free energy was irremediably becoming exhausted. We do not know how energy is built up again in the physical universe; but the analogous process in the domain of organic evolution would seem to be paedomorphosis. A race may become rejuvenated by pushing the adult stage of its individuals off from the end of their ontogenies, and such a race may then radiate out in all directions . . . until racial senescence due to gerontomorphosis [see below] sets in again.' [4]