While details on sea level changes are relatively recent, biogeography dates back to the 18th and 19th centuries, and was given particular impetus by the man who arrived at the theory of evolution at much the same time as Charles Darwin: Alfred Russel Wallace. And much as Darwin was influenced by his findings on the Galapagos, Wallace drew on discoveries he made on tropical islands, where sea level changes played vital roles in determining species distribution.

Though best known as a co-discoverer of evolution who gained far less renown than Darwin, Wallace was an astonishing man: an explorer, self-taught naturalist, discoverer of new species, a prolific - and outstanding - writer, opponent of eugenics and believer in both women's rights and spiritualism. It's surprising, then, that historian Dr John van Wyhe has lately downplayed some of his achievements - to the great annoyance of entomologist Dr George Beccaloni.

"This is of concern to myself and other Wallace scholars, as there is a danger that van Wyhe's errors will end up being accepted as fact by others," Beccaloni informed me by e-mail. Beccaloni works in London's Natural History Museum, and in 1999 set up the A. R. Wallace Memorial Fund.

Wallace was born in Wales in 1823, and during his childhood surely seemed an unlikely future hero. Financial troubles led to him being withdrawn from grammar school, and he became an apprentice surveyor. In his early twenties, Wallace met entomologist Henry Bates and began collecting insects. This, plus reading books by travelling naturalists such as Darwin, led Wallace to partner Bates in an expedition to Brazil, where they aimed to collect animal specimens that could be sold in Britain.

After three years, Wallace headed back home, nearly losing his life as the ship he was on caught fire. He wrote scientific papers, contacted naturalists including Darwin, and then embarked on his great exploration of the Malay archipelago that today comprises Singapore, Malaysia and Indonesia.

He later related his travels and discoveries in a now classic book, . This covers adventures in which he was invariably the sole European, making dangerous sea crossings, suffering occasional injuries, going to bed beneath shrunken human heads in a Borneo hut and unwittingly sleeping soundly below a four-metre python in Sulawesi.

Wallace was financing his travels by collecting specimens, and recounted killing several orang-utans. Though he clinically told of how they might require a few shots to dispatch, he also became very fond of a baby he found and kept as a pet. Though saddened when it died of fever, Wallace preserved its skin and skeleton for shipping back to Britain.

As he journeyed, noting variations in his specimens between individuals within species, and discovering a host of creatures such as a frog that could glide through the air, Wallace pondered ongoing debates on whether species were fixed - immutable - or could change over time.

While recovering from a bout of fever on islands east of Sulawesi, Wallace wrote a now famous letter to Darwin, together with a 20-page essay, . In this, Wallace asserted that should physical conditions change, there would be a change towards a superior variety that "would be in all respects better adapted to secure its safety, and to prolong its individual existence and that of the race".

Evidently, Darwin was startled by the essay, which outlined ideas similar to his own. It was soon published as part of a joint paper with Darwin, who was prompted to set down his ideas in his monumental work, .

Though Darwin's name has become tied to evolution, Wallace did make another major discovery. During a brief stay on Bali, he found birds much as on Java. Making a sea crossing of less than 20 miles to the east, to Lombok, he was surprised to encounter "a totally different set of species", such as cockatoos. Beyond, in Timor, there was a higher proportion of birds related to Australian forms. There were similar differences in creatures such as insects.

Wallace speculated on previous linkages between islands, such as whether Timor had been connected to Australia. Later, scientists appreciated that when sea levels fell during ice ages, islands as distant as Bali became part of the Asian mainland - allowing the spread of animals like tigers. But the deep Lombok Strait was always sea, and proved a formidable barrier. Today, biologists worldwide are familiar with the "Wallace Line" that runs through the strait and north between Borneo and Sumatra.

To the west, Wallace noted the animals and even people belonged to the Indo-Malayan region; eastwards they were more Australian. There isn't a hard and fast line, but islands including Lombok and Sulawesi are in a transitional zone that's one of the most fascinating areas on earth. It bears a name that's a fitting tribute to this giant among naturalists: Wallacea.