Alfred Russel Wallace: Architect of Evolution

Nothing daunted, Wallace set off again - this time for Indonesia. He was to spend eight years there, undertaking 70 different expeditions and travelling 14,000 miles between the islands. In all, he collected 125,660 specimens - which included more than a thousand species that were new to science.

primates.com Orangutans gave Wallace much food for thought

From Singapore, Wallace first investigated the coast of Borneo, where he was the first European to study its population of orangutans. Then he sailed east to Bali, and on the Spice Islands and New Guinea. On the nearby Aru islands, he was the first naturalist to capture exotic birds of paradise.

During his four years in the region so far, Wallace had come to realise that the western part of Indonesia had vegetation and animals like those found in Asia; while the eastern part was populated by creatures and plants similar to Australian species. The dividing line between the two is still called the Wallace Line.

In February 1858, Wallace was laid up by malaria on the island of Ternate and had time ponder on these observations - leading to his great scientific breakthrough. In the background of his mind was a treatise on human populations, written by the English political economist Robert Malthus.

Malthus had said human populations are limited by the amount of food available to them. Once a community started to outstrip its food supply, there was a "struggle for existence".

Wallace recognised the equivalent struggle for existence in the jungle. He realised that if the offspring of a species happened to have a variation that made them better able to obtain food - or to avoid becoming food for a predator - then they would survive better. And they would pass on that variation to their offspring. In this way, any changes that improved survival would be enhanced, and gradually make the succeeding generations grow away from the original ancestor.

Tropical islands inspired Wallace's Theory of Evolution

In an excited rush, Wallace wrote up his theory in a paper called On the Tendency of Varieties to Depart Indefinitely from the Original Type. It concluded: