The Ghost in the Machine (25 page)

 

 

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]

 

 

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]

 

 

Neoteny in itself is of course not enough to produce these
evolutionary bursts of adaptive radiations. The 'rejuvenation' of
the race merely provides the opportunity for evolutionary
changes to operate on the early, malleable phases of ontogeny:
hence paedomorphosis, 'the shaping of the young'. In contrast to
it, gerontomorphosis (
geras
= old age) is the modification of
fully adult structures which are already highly specialised.* This
sounds like a rather technical distinction, but it is in fact of vital
importance. Gerontomorphosis cannot lead to radical changes
and new departures; it can only carry an already specialised
evolutionary line one more step further in the same direction --
as a rule into a dead end of the maze. To quote de Beer again:

 

The terms gerontomorphosis and paedomorphosis, therefore, express
not only the stage in the life history of an animal with which they
are concerned, but they also convey the meaning of racial senescence
and rejuvenescence. It is interesting to note that as a result of
considerations based on a different line of thought, Child had been
led to express similar views. [5]  'If evolution is in some degree a
secular differentiation and senescence of protoplasm, the possibility
of evolutionary rejuvenescence must not be overlooked. Perhaps the
relatively rapid rise and increase of certain forms here and there
in the course of evolution may be the expression of changes of this
sort.' [5a]
* The word 'gerontomorphosis' was coined by de Beer as a contrast
to Garstang's 'paedomorphosis'.

 

 

Draw Back to Leap

 

 

It seems that this retracing of steps to escape the dead ends of the
maze was repeated at each decisive evolutionary turning point. I have
mentioned the evolution of the vertebrates from a larval form of some
primitive echinoderm. Insects have in all likelihood emerged from a
millipede-like ancestor -- not, however, from adult millipedes, whose
structure is too specialised, but from its larval forms. The conquest of
the dry land was initiated by amphibians whose ancestry goes back to the
most primitive type of lung-breathing fish; whereas the apparently more
successful later lines of highly specialised gill-breathing fishes all
came to a dead end. The same story was repeated at the next major step,
the reptiles, who derive from early, primitive amphibians -- not from
any of the later forms that we know.