The Faber Book of Science (47 page)

One group is of spindle-shaped cells very much like the sperm of the higher animals. The other is a sort of egg considerably larger than the sperm because, like most eggs, it contains a rich store of reserve food to nourish a growing ‘embryo.’ Sooner or later a sperm cell will seek out an egg, the two will fuse, and thus they will pool the hereditary characteristics carried by the sperm and by the egg. Thus Volvox introduces a method of procedure almost universal among the higher plants and animals. Moreover, there is even the beginning of the distinction between Male and Female
individuals
as well as a distinction in the sex cells themselves, because, in the species which I have been observing, a given individual usually produces either eggs only or sperm only. The cynic who said that the two great errors in creation were the inclination of the earth’s axis and the differentiation of the sexes probably had no idea how long ago the second error was made. To eliminate it we would not have to wipe the slate quite clean but we would have to go back at least as far as Volvox.

Being the inventor of sex would seem to be a sufficient distinction for a creature just barely large enough to be seen by the naked eye. But as we have already said, Volvox brought Natural Death as well as Sex into the world. The amoeba and the paramecium are potentially immortal. From time to time each divides itself into two, but in the course of this sort of reproduction no new individual is ever produced – only fragments of the original individuals, whose life has thus been continuous back to the time when life itself was first created. Though individuals can be killed there is no apparent reason why amoeba should ever die.

Individuals have been kept alive in laboratories for years by carefully isolating one-half of the organism after each division. What memories an amoeba would have, if it had any memories at all! How fascinating would be its firsthand account of what things were like in Protozoic or Paleozoic times! But for Volvox, death seems to be as inevitable as it is in a mouse or in a man. Volvox must die as Leeuwenhoek saw it die because it has had children and is no longer needed. When its time comes it drops quietly to the bottom and joins its ancestors. As Hegner, the Johns Hopkins zoologist, once wrote: ‘This is the first advent of inevitable natural death in the animal
kingdom and all for the sake of sex.’ And as he asked: ‘Is it worth it?’

Nature’s answer during all the years which have intervened between the first Volvox and quite recent times has been a pretty steady
Yes
…

No detectable difference in structure exists between two conjugating protozoa. There are no males and no females. Neither are there any specific sex cells – no eggs and no sperm. It is in respect to these two facts that Volvox takes a great step toward sexuality as it is commonly known.

Volvox never indulges in the kind of conjugation we have been describing. Neither does it ever divide into two halves. Instead it produces what may properly be called ‘children’ – sometimes by the vegetative process which Leeuwenhoek described and sometimes by a more surprising method which no free-living, single-celled animal ever practices.

Somewhere inside its sphere appear certain groups of small cells which might at first sight be mistaken for vegetative buds. But they develop quite differently in either one of two ways. Sometimes they become quite large. Sometimes on the other hand they split up into a great number of extremely small mobile cells. The first are eggs; the second sperm. Neither is good for anything by itself. But each is ready to do what the egg and the sperm of all the higher animals do. The egg waits. The sperm seeks it out. Then the two fuse and the fertilized egg is endowed with the hereditary traits contributed by the sperm as well as with those originally its own. Some species of Volvox are hermaphroditic as many lower organisms are. A single individual, that is to say, produces both male and female cells. But Volvox is also inventing the sexual differentiation of the whole organism. Certain species are commonly either male or female; the one producing only eggs, the other only sperm.

In what ways, one is bound to wonder, is this differentiation of sex cells and the further differentiation of male and female organisms superior to the simpler arrangement which the protozoa have managed to get along with during millions of years?

It has been, as satirists have so frequently pointed out, the cause of a lot of trouble in the world. Yet there must be compensating advantages, because as one moves upward along the evolutionary scale sexuality becomes universal and even hermaphroditism tends to disappear.

That sexual differentiation provides a richer emotional experience is
a reason that few biologists are likely to admit as relevant and indeed it would be hard to prove that Volvox finds life more colorful than a paramecium does. Hence the biologist has to fall back upon such things as the superior viability of an egg, which can be heavy with reserve food resources because it does not have to be active when a small mobile sperm is there to seek it out. Possibly another fact is even more important. In all the higher animals sperm and egg cells differ from every other cell in the body of that organism in that they have only half the normal number of chromosomes and that the normal number is re-established when the sperm’s half-number is added to the egg’s half-number – which arrangement certainly shuffles hereditary characteristics more thoroughly than when the offspring has the whole inheritance from both sides of the family. In any event (and to repeat) there must be some advantage, since every animal above the protozoan level tends to adopt the novel arrangement first observable in Volvox.

In his account of his first discovery of Devonian fishes in the Old Red Sandstone, Hugh Miller describes how he split open a calcareous nodule and found inside ‘finely enamelled’ fish scales. ‘I wrought on with the eagerness of a discoverer entering for the first time a
terra
incognita
of wonders. Almost every fragment of clay, every splinter of sandstone, every limestone nodule contained its organism – scales, spines, plates, bones, entire fish … I wrought on until the advancing tide came splashing over the nodules, and a powerful August sun had risen towards the middle of the sky; and were I to sum up all my happier hours, the hour would not be forgotten in which I sat down on a rounded boulder of granite by the edge of the sea and spread out on the beach before me the spoils of the morning.’ This August day was in 1830. The young man’s hammer had discovered the remains of the earliest fishes, the
Ostracoderms
whose leathery skins were armoured with plates and spines, and who, lacking a jaw, fed through a slit set below the pointed snout. The Devonian seas were full of these creatures.

Occasionally, when an inland sea dried up, there must have been a horrible flapping and floundering, a dull rattling of horny armour before they suffocated and the bodies of untellable shoals were buried, later to form a dense mass of fossilized remains.

Such happenings, however, were no more than local catastrophes, for elsewhere these vertebrates and their successors, so crucial in the evolution of species, throve and multiplied to such an extent that the
Devonian is sometimes called the Age of Fishes. By the middle of the period as well as the
Ostracoderms
(many would wish to withhold the name of fish from an animal that could not open its mouth) there were more developed fishes of many kinds, some of them already wearing scales. A few species such as
Dinichthys
grew to as much as twenty feet and had heavily armoured jaws as ruthless as a mechanical excavator. It is true that before them the eighteen-inch trilobites, the six-foot arachnids, had their relative power to tyrannize, but it seems that these great predatory vertebrates must have brought the first keen fear into the sea. Something akin to human emotion ran along those newly evolved spines when
Dinichthys
hurled himself among the helpless shoals.

Among the scaled fish one Devonian group seems to have held the secret of the future. These were the varieties that had paired fins and lungs enabling them, if stranded by seasonal drying, to shuffle back to the water. From them, so far as we know, is descended the whole train of the land vertebrates.

Already before the close of the Devonian Age, the land had taken the place of the seas as the stage on which the great scenes of evolution were to be played. Algæ and seaweed had already breathed out the free oxygen that made life on land possible. With this invisible atmospheric envelope of the earth ready to receive it, life came up from the sea. The lunged fish had given rise to true amphibians; all manner of insects, not yet able to fly, had crawled on to the land, and there were millipedes, mites, and spiders. The land that had always been silent and undisturbed began not only to be minutely stirred by small burrowings and by the growth of plants, but was marked by the impress of feet, even though between the footsteps went the groove of a scaly tail.

The country which the eyes of these amphibians saw sharply if vacuously was already green. With a virgin environment to exploit, the new land plants flourished amazingly. They were of those smooth, spiny and militant kinds we have come to associate with tropical conservatories, but already they had much in common with modern plants; sap flowed in them and they breathed through open pores. Indeed, by the end of the age the vegetation had developed far towards the luxuriance of the Carboniferous forests. There were the
fountain-like
tree ferns, and seed ferns carrying little nuts below their fronds; the big horsetails had a tree growth and there were even forerunners of
true conifers. All these forms are extinct, yet they were so near to what has become familiar that I doubt whether the ordinary, unobservant passer-by would notice them if they could spring up again in hedgerow or wood.

In no geological scheme is the Devonian accepted as a major turning-point; it does not mark either the beginning or the end of one of the great eras. To me, in this effort of recollection, it appears to be one. However broken up and unrecognizable, some of the land that was to be Britain was clear of the sea and green with vegetation. The main masses of our mountains had been formed, and the Old Red Sandstone was ready to support heavy cornfields and cider orchards. To watch the close of a Devonian day would not have been the unimaginable experience of a few hundred million years earlier. As the shadows of the trees lengthened there would have been a clapping and harsh rustling of the big leaves on the river bank as clumsy animals pushed among them; if there was no bird-song or even the humming of insects at least there was that most characteristic evening sound, the occasional splash of fish in quiet water.

Perhaps more than any other, the age that followed was to reach through time and effect the face and fortune of the British Isles. This it was to do by creating a substance – coal – which at a certain moment in their historical evolution men sought as eagerly as food, so eagerly that they were ready to leave their habitat and become pale-skinned burrowing creatures, coming to the surface only at night. To move away from the pleasanter places and huddle their dwellings round the grimy entrance to their tunnels.

At first, with some spread of warm and shallow seas, limestone formed, the Carboniferous or Mountain Limestone that was to be built into some of the most solid and respectable piles in England, buttresses of its pride and self-confidence. The work of silting up these Carboniferous seas was completed by deposits brought from the northern continent of Atlantis, then hot, mountainous, and swept by monsoons. A large river with tributaries drawn from territories stretching from the north of Scotland to Norway poured out its coarse sediments across north-eastern England. So were Norwegian pebbles brought to Yorkshire and held in the Millstone Grits that were laid down as the deltas of this northern river. Silting, combined with the elevation of expanses of low-lying land and the influence of the warm rains of the southern monsoons, led to the formation of marsh and
brackish swamps where the Coal Measure forest grew in sombre luxuriance.

It is sombre in these swamps, for the foliage is dark green and there are nowhere any flowers. Yet there is scent in the air. Here already is the rich aromatic breath of resins, a presage of the smell of pinewoods on summer days when pine cones crack in the sun. In many places the trees grow straight from the tepid water that carries a dull film where clouds of pollen have blown across it. Ferns feather the mud-banks and there are thickets of horsetails with the radiate whorls and neatly socketed stems of their diminished and weedy descendants. When, as a very small child, I was playing with a horsetail that had been growing as a weed in one of our flower-beds, dismantling it section by section like a constructional toy, I remember how my father told me it was one of the oldest plants on earth, and I experienced a curious confusion of time. I was holding the oldest plant in my hand, and so I, too, was old. Now huge horsetails are growing in the Carboniferous swamp while above them the fern trees with their sprouting leaves cut off most of such sunlight as has succeeded in straying through the still loftier canopy of the scale trees – the lycopods whose slender trunks are chequered like snake skin. Across the hundred-foot verticals of the growing scale trees are the diagonals of many that have fallen and lodged against their fellows, while others lie horizontal, already
half-digested
by the swamp. Here decay is active among growth, trees and ferns thrusting towards the summit of their life, while others are slowly reverting to inorganic forms.

Among these imperceptible rhythms of growth and decay are the quicker movements of the swamp creatures. There are shoals of fish in the pools and slow streams of the forest; vast beds of molluscs line the edges of the lagoon. Dragging their wide bellies across the mudbanks, sagging heavily back into the water, go amphibious monsters like grosser crocodiles. Over the streams and pools, through the oppressive greenish light, with a clittering of glassy wings, twist gigantic dragonflies, the largest insects the earth will ever know.

There is still no spring in these forests, for all the foliage is evergreen, no seasonal rise and fall but only, continuously, life going on beside decay. The toll of decay mounts with the centuries, the swamp lives above a tremendous accumulation of its own past, tree-trunks, leaves, and fronds, and scattered among them the broken bodies of the animal population – bones, empty shells, the wings of dragonflies.

The swamp itself mounts slowly, but meanwhile the whole platform of land is sinking until somewhere far away the sea breaks in, sea water invades this stagnant world, fishes choke, the amphibians, if they can, move away and the insects go – as insects do. For a time forlorn, ragged trunks of dead scale trees stick through the water. But they sink, the whole scene sinks and the particles of sediment begin to fall again burying all the dead stuff of the swamps and forests in layers of forgetfulness. It is a drowsy scene to contemplate, and sleep muffles me. I see
Loxomma,
the amphibian, his flesh fallen away to reveal the long column of his spine and the little bones of his hands and feet. The spine is lengthening, vertebra after vertebra, without end, and running through the vista of their bony arches there is a mounting current, a sense of the passage of some energy and power. The vista of arches – I see now that it is a tunnel and that there are living creatures crawling along it, each with a single eye shining in its head. I am stupid, they are only lamps, and the roaring in my ears is nothing but a drill, one of those confounded drills. ‘Christ, look at the old blighter,’ some one says, and I notice that
Loxomma
is there again (perhaps he had never gone) and they have excavated him with their drill. ‘Makes your spine creep a bit, don’t it? Christ, look at that hand …’