A mystery story lies at the heart of the late Loren Eiseley's last book - actually a mystery within a mystery. After some detective work in 140-year-old journals, Eiseley discovered a previously unrecognized source for one of the central ideas in Darwin's theory of evolution - the idea of natural selection. That discovery opened up an even deeper mystery: Why Darwin, who has often been praised for "his magnanimity, his friendliness, his lack of pretense," would deny a fellow scientist due credit for a theory without which the whole theory of evolution would disintegrate.

Darwin, you will recall, made some disturbing observations in 1832 during his long journey as resident naturalist on the H.M.S. "Beagle." For example, the varied shapes of beaks on finches in the Galapagos Islands, where finches had a sort of monopoly on the bird business: Why would a finch develop a beak like a parrot? Simply because there were no parrots around to compete, and a properly equipped finch culd make a good living on what would normally be parrot food. Darwin went home to think it over and work up more information and some theories. He was still thinking a quarter of a century later.

Today, Darwin's finches are probably as famous as Newton's apple - and somewhat better authenticated. In a sense, they played contradictory roles in the history of the human mind. Newton's "Principia" took a world of apparently random or semimiraculous events, traced the mathematical logic underlying these events and made them measurable, predictable, rational. He was the father of the orderly, conservative, rather smug world-view of the 18th century.

Darwin, in contrast, plunged our minds into a world where a reptile could develop into a bird - and that's not all. By changing the universe from a static to a dynamic model, he certainly helped produce a situation in which men could fly and atoms could split. To him, as much as to any man, we owe the extential anxiety, the permanent culture-shock that is now our natural habitat.

He could hardly have done it alone. His most important predecessors have long been well-known and duly honored. They include his own grandfather, Erasmus Darwin, and Jean Baptiste de Lamarck, who toyed with the idea of evolution in an earlier generation but didn't produce a complete, workable theory. There was Thomas Malthus, whose study of population codified the idea of the struggle for existence, and Sir Charles Lyell, whose study of geology changed the time-frame in which scholars thought about world history, providing the eons necessary for evolution to take place. Above all, therew was Alfred Russel Wallace, who worked out his own independent theory of evolution, sent his work to Darwin (not knowing they were both working the same side of the street), and thus forced Darwin to rush into print in 1958 with "On the Origin of Species."

Iseley's most significant contribution in this book is that he adds the name of Edward Blyth to that distinguished list. The honor to Blyth is more than a century overdue, and if blame must be assigned for this long dealy, it must be assigned to Darwin. It is not that Darwin left Blyth unmentioned entirely, but he praised him for the wrong things - small observations of relatively unimportant details rather than a key theory in the total system.

They knew one another. Blyth called Darwin "a very clever, odd, wild fellow," and Darwin said that Blyth "will never do what he could do, from not sticking to any one subject." Both were right, and there is some justification for Darwin's decision not to give Blyth the credit (which Blyth never publicly claimed) for inventing the idea of natural selection.

Eiseley demonstrates with the thoroughness of a detective or a prosecuting attorney that Blyth deserves such credit - and he reprints the articles, from the Magazine of Natural History, 1835 to 1837, in which Blyth clearly made an impression on the mind of Darwin. He then examines a variety of theories on why Darwin never acknowledges his debt to Blyth. These range from a theory of unconscious creation (in which Darwin's mind is compared to that of Coleridge) to the obvious explanation that Darwin wanted to keep the glory of his discovery to himself.

There are other points. Although he formulated the idea of natural selection, Blyth was not an evolutionist; on the contrary, he believed that natural selection helps to preserve the identity of a species, as it does in relatively short periods and restricted geographical limits. He toyed with the idea but did not develop it into a system or sell it to the world, as Darwin did. Indeed, decades later, he may not have remembered even working it out.

While exploring the fascinating, untold story of Blyth, Eiseley enriches the book with his own reflections on evoluation and - perhaps most absorbing of all - plunges the reader into the scientific world of the m-d-1800s, where the amateur could still make an impact and a variety of ideas on the development of man were competing for attention - like so many now-extinct animal species that once competed for food.

The journey is as strange as Darwin's was on the "Beagle," and the book is a significant addition in a much-explored field.