The text, in an easy to read two-column layout, is a continuous and quite substantial narrative, broken up only by boxes with short quotes from Wallace's own writings. The focus is on the two collecting trips which were central to his career, to the Amazon and the Malay Archipelago, and there's enough excitement here that Dorion wisely eschews dramatisation. Also touched on are Wallace's broader life as a scientist: his formative childhood experiences, his relationships with Darwin and other naturalists, and so forth.
Threaded through this is an account of Wallace's developing thinking on species and their distribution. There's no direct attempt to explain natural selection, but Darwin's Rival gives a good feel for the biogeographical logic that led both Wallace and Darwin to the idea. ("Every species has come into existence coincident both in space and time with a pre-existing closely allied species.") There is nothing on Wallace's social activism or nascent environmentalism, or his later involvement with spiritualism. Nor is there much general background on Brazil or the Dutch East Indies, or on Victorian England for that matter.
The immediacy and impact of the text are powerfully enhanced by Tennant's varied and striking illustrations. His palette is muted and the use of colour wash favours impression over detail, but both are good matches for the content and scale. Many of the illustrations extend the full width of two pages, nearly two thirds of a metre across. Sometimes this is dramatic: a depiction of the shipwreck in which Wallace lost most of his Amazon collections and nearly his life, for example, or a panorama of the Borneo jungle. There are some effective maps, such as a full-width map showing his route up the Amazon, half a page high with text below, and a full two-page map of the Malay archipelago, showing Wallace's routes and overlaid with illustrations of animals he encountered. And sometimes the space is filled with lots of smaller drawings: one two-page spread conveys the diversity of beetles Wallace collected, another the variety of his collecting equipment.
The overall effect is engaging, informative, and elegant. The most obvious audience of Darwin's Rival may be children between 9 and 12, but my seven year old and I both really enjoyed it; it is simple and straightforward but not at all dumbed down.