During the next three years the Beagle sailed down the South American east coasts, rounded cape horn and came north again along the coast of Chile. The expedition then then sailed away from the coast six-hundred miles into the Pacific Ocean until they reached the lonely archipelago of the Galapagos Islands. Here, Darwin's questions about the creation of Species recurred, for in these island he found a fresh variety. He was fascinated to discover that the Galapagos animals bore a great resemblance to the creatures he had seen back on the mainland, but differed to them in detail. There were cormorants, black long- necked diving birds similar to those that fly low along Brazilian rivers, but here on the Galapagos, their wings were so small and with such stunted feathers that they had lost the power of flight. There were iguanas, large lizards with a crest of scales along their backs. Those on the continent ate leaves and climbed trees but on the Galapagos, there was little vegetation, one species fed on seaweed and clung to rocks among the surging waves with unusually long and powerful claws. There were tortoises, very similar to the mainland forms except that these were many times bigger, giants that a man could ride. The English vice-governor of the Galapagos Islands told Darwin that even within the archipelago, there was noticeable variety. The tortoises on each island were slightly different, so that it was possible to tell which island that they came from. Those that lived on relatively well-watered islands where there was ground vegetation to be cropped, had a gently curving front edge to their shells just above the neck. But those that came from arid islands and had to crane their necks in order to reach cactus plants or leaves off trees, had much longer necks and a high peak to the front of their shells that enabled them to stretch their necks almost vertically upwards.
The suspicion grew in Darwin's mind that species were not fixed forever. Perhaps one could change into another. Maybe, thousands of years ago, birds and reptiles from continental South America had reached the Galapagos, ferried on the rafts of vegetation that float down the rivers and out to sea. Once there they had changed, as generation succeeded generation, to suit their new homes until they became their present species.
The differences in them and their mainland cousins was only small , but if such changes had taken place, was it not possible that over many millions of years, the cumulative effects on a dynasty of animals could be so great that they could bring about major transformations? Maybe fish had developed muscular fins and crawled on to land to become amphibians; maybe amphibians in their turn had developed water-tight skins and become reptiles; maybe even some ape-like creatures had stool upright and become the early ancestors of man.
In truth, the idea was not a wholly new one. Many others before Darwin had suggested that life on Earth was interrelated. Darwin's revolutionary insight was to perceive the mechanism that brought these changes about. By doing so, he replaced a philosophical speculation with a detailed description of a process, supported by an abundance of evidence, that could be tested and verified; and the reality of evolution could no longer be denied.
His fundamental argument was this. All individuals of the same species are not identical. In one clutch of eggs from, for example, a giant tortoise, there will be some hatchlings which, because of their genetic make-up constitution, will develop longer necks than others. In times of drought they will be able to reach leaves and so survive. Their brothers and sisters with shorter necks will starve and die. So those best fitted to their surroundings will be selected and be able to transmit their genetic characteristics on to their offspring. After a great number of generations, tortoises on the arid islands will have longer necks than those on the watered islands. And so one Species will have given rise to another.
This concept did not become clear in Darwin mind until long after he had left the Galapagos Islands. For twenty-five years he painstakingly amassed evidence to support it. Not until 1859, when he was forty-eight years old, did he publish his idea and even then he was only driven to do so because another younger naturalist, Alfred Wallace, working in South-east Asia had formulated the same idea. He entitled the book in which he set out his theory in detail “The Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life”.
Ever since then, the theory of natural Selection has been debated and tested, refined, qualified and elaborated. Later discoveries about genetics, molecular biology, population dynamics and behaviour have given it new dimensions. Nevertheless, it remains the key to our understanding of the natural world and it enables us to recognise that life has a long and continuous history during which organisms, both plant and animal, have changed generation by generation as they colonised all parts of the world.
The direct, if fragmentary evidence for this history lies in the archives of the earth, the sedimentary rocks. The vast majority of animals leave no trace of their existence after their passing. The flesh decays, their shells and their bones become scattered and turn to powder. But very occasionally, one or two individuals out of a population of several thousand have a very different fate. A reptile becomes stuck in a swamp and dies. Its body rots but its bones set into the mud. Dead vegetation drifts to the bottom and covers them. As the centuries pass and more vegetation accumulates, the deposit turns to peat. Changes in sea level may cause the swamp to be flooded and layers of sand to be deposited on top of the peat. Over great periods of time, the peat is compressed and turns to Coal. The reptile's bones still remain within it. The great pressure of the overlying sediments and the mineral-rich solution which circulate through them cause chemical changes in the calcium phosphate of the bones. Eventually they are turned to stone, but they retain not only the outward shape they had in life, albeit sometimes distorted, but on occasion even their detailed cellular structure is preserved so that you can effectively look at sections of them through a microscope and plot the shape of the blood vessels and the nerves that once surrounded them.
The most suitable places for fossilisation are in seas and lakes where sedimentary deposits like sandstones and limestones are slowly accumulated. On land, where for the most part rocks are not built up by deposition but broken down by erosion, deposits, such as sand dunes, are only only very rarely created and preserved. Consequently the only land-living creatures likely to be fossilised are those that fall into water and die. Since this is exceptional fate for most of them, we are never likely to know from fossil evidence anything approaching the complete range of land creatures that has existed in the past. Water-living animals such as fish, molluscs, sea urchins, and corals are much more promising candidates for preservation. Even so, very few of these perished in the exact physical and chemical conditions necessary for fossilisation. Of those that did, only a tiny proportion happen to outcrop on the surface of the ground today; and of these few, most will be eroded away and destroyed before they are discovered by fossil hunters.
