Berry begins with a biographical sketch built around excerpts from Wallace's autobiography. He was born in 1823 into genteel poverty, had to earn a living through his collecting, and was only saved from poverty by the grant of a pension in 1881. He died in 1913.
Roughly half of Infinite Tropics is devoted to Wallace's scientific work, including the full text of three of his most important papers. He first showed his power as theoretician of evolution in the 1855 "Sarawak Law" paper, which sketches an argument that "Every species has come into existence coincident both in time and space with a pre-existing closely allied species". It was the 1858 "Ternate" paper, sent to Darwin, which sparked the famous presentation of natural selection at the Linnean Society. And "The Origin of Human Races" linked culture, races, and human evolution.
Other material includes more on evolution — disagreement with Darwin about the role of sexual selection, mimicry and species definitions in the lepidoptera — biogeography — Amazonian rivers as boundaries, the Wallace Line dividing the Indonesian archipelago between Australasian and Asian flora and fauna — general natural history — encounters with jaguars, orangutans and birds of paradise — ideas about conservation, and more. This reveals Wallace as both an astute observer and a deep thinker, with ideas that were often ahead of their time.
Much harder for the modern reader to sympathise with is Wallace's fervent spiritualism and endorsement of phrenology. Berry's extracts illustrate his conversion and the way in which his spiritualism influenced his science, leading him to a more teleological view of evolution. (As a curiosity, we learn that when an American medium was prosecuted for fraud in 1876, Darwin contributed £10 to the prosecution while Wallace appeared as an expert witness for the defence.)
Some of the most dramatic passages in Infinite Tropics are descriptions of the hazards and excitements of Wallace's collecting expeditions to the Amazon and to the Malay Archipelago. Wallace lost all his Amazonian specimens and barely survived himself when his ship sank in the Atlantic. Travelling by himself and without logistical support, he was much closer to the local people than travellers such as Darwin, and overcame some of the racial and cultural prejudices of his time. And his observations on the more mundane aspects of travel are entertaining and insightful.
Wallace also became a confirmed socialist, though one who took his own path rather than following any theorist or party. The extracts included illustrate his often idiosyncratic ideas about inheritance and private property, public education, strikes, imperialism, and so forth.
In a final coda Berry compares Wallace's obscurity to Darwin's fame. His wrong-headed ideas on several topics, his failure to produce as compellingly presented an argument as The Origin of Species, and his obstinacy and lack of tact were probably larger contributors to this than his lack of social standing.
Wallace's style is straightforward and pleasant to read, and Berry has skillfully selected passages. The result in Infinite Tropics is a portrait of a life that is intriguing in its own right but which also offers insights into Victorian science and its complex connections to politics and religion.