As we wander the natural world, from mud flat to mountain top, from river bed to rocky outcrop, the life that we encounter falls into readily recognizable forms or, as we know them now, species. The similarities and differences between species help even the layman to recognize the extent of their relationship.

Bust of Charles Darwin

Even to the uninformed eye, the various diving and dabbling ducks I wrote about last month are clearly more similar, more closely related to each other, than they are to geese or swans.

Similarly, on seeing various kinds of swans the novice will know them for swans and will not confuse them with other, less closely related groups such as hawks, flamingos or storks, for example.

The process that has brought about Earth's vast array of living forms, which weigh in today at 30-100 million species, we know now as "evolution."

I'll bet that on reading the word evolution, at least seven out of 10 readers have already added in their own minds: "Darwin." The theory of evolution is so closely linked with Darwin that it is frequently referred to as Darwinism.

Fame and glory come when least expected. Charles' father, Robert Darwin, once said to him, "You care for nothing but shooting, dogs and rat-catching, and you will be a disgrace to yourself and all your family."

Who was this Charles Darwin who so famously proved his father wrong, and how did this monumental theory become so closely entwined with his name?

First we should acknowledge that Darwin was not the first to think of evolution; a number of scientists before him had also considered the idea. What Darwin did was give it a mechanism. This was what his predecessors had failed to find. Darwin's answer was what he called "natural selection."

Evolution -- the process of change in existing species and the appearance of new ones -- has been occurring since life began on Earth more than 3 billion years ago, resulting in the millions of species alive today and the immensely greater number that became extinct along the way.

In each generation, many more organisms are born, hatched, germinated or budded than can possibly survive. Natural selection among this excess of potential occurs because some varied traits, which arise by chance, confer survival advantages on a particular organism in a particular environment.

Thus, as one example, a wide-ranging lizard may by chance produce some offspring with tails longer than normal. If those longer tails improve the lizard's balance while running on thin branches, then the long-tailed offspring are likely to survive better in trees than their shorter-tailed relatives. In a forest habitat, long-tailed lizards would be likely to leave more offspring than those with shorter tails, and the genetic tendency toward long tails will spread through the population.

Over time, the longer-tailed lizards are most likely to prevail in an ecological niche that has trees with many thin branches. The shorter-tailed lizards may die out as a result of the competition, or they may prevail in a different niche.

In this way, populations eventually diverge into different species or are forced toward extinction through competition with populations better adapted to the ecological circumstances.

In the 19th century, Victorians and Christians tended to view evolution as a kind of ladder that led not just from the simpler to the complex, but from the imperfect to the perfect. We perceive ourselves as having been elevated via that moving ladder to the pinnacle of evolution, as if we were on some lofty summit of nature where perfection was synonymous with humanity.

Yet not only is there no such ladder, but the process of evolution by natural selection is a process entirely without preplanning -- it just happens.

We share genes consisting of DNA even with bacteria, and our own lineage converges with that of the bacteria if we look far enough back.

Darwin's genius was to recognize the process by which life endlessly diverges and to explain how something as simple as natural selection could have led to the astonishing biodiversity that exists on our planet.

Charles Darwin was born in Shrewsbury, England, in 1809, son of the sometimes exasperated doctor Robert Waring Darwin and his wife Susannah, and grandson of the scientist Erasmus Darwin and the potter Josiah Wedgwood.

Charles' was a relatively privileged, educated background. He began by studying classics at Shrewsbury, then medicine at Edinburgh, but medicine was not to his taste and he was sent to Christ's College, Cambridge, to study theology in 1827.

Instead, however, his interests led him to collect plants, insects and geological specimens. His scientific inclinations were encouraged by his botany professor, John Stevens Henslow, who sent him to Wales to learn geology field methods. It was Henslow who recommended him for a place as a naturalist and companion to the captain of HMS Beagle on its round-the-world surveying expedition (1831-36).

After four years spent exploring the geology and wildlife of South America, Darwin visited the Galapagos Islands, the source, he later maintained, of all his ideas.

Fresh from travels in South America, he was not greatly impressed by the stark landscapes of the islands, nor by the predominantly reptilian fauna and the Spartan flora.

He was struck, instead, by the fact that most of the plants and animals he found confined to the islands, although clearly related to species in South America, were different. Not just different, they were unique, occurring nowhere else in the world.

We must remember that Darwin was not only born and educated in an era dominated by a belief in creationism, but that he had trained for the clergy.

Nevertheless, from his geological training he had to recognize the volcanic nature of the Galapagos, and the fact that they were therefore newer than much of the rest of the world.

Here was someone who understood the religious and social depths of creationism, but his firsthand experience exposed him to one of the greatest questions of his period. If all creation had taken place at the beginning of the world, and given that the Galapagos Islands did not erupt into existence until very much later, how was it that unique, endemic species occurred there?