The numbers from Aru, reflecting a ratio of specimens to species of almost six to one, signal a critical fact about Alfred Wallace and the way he worked. Being a commercial collector as well as a natural historian, he wanted multiple specimens of a given species, not just one or two representatives, especially if the species was visually impressive, such as the birdwing butterflies, the giant longicorn beetles, or the birds of paradise. In the Amazon he had taken 12 specimens of a spectacular flame red bird, the Guianan cock-of-the-rock (Rupicola rupicola), and admitted he would have killed 50 if they hadn't been so rare and elusive. In Aru, likewise, he was greedy for as many specimens of the greater bird of paradise (Paradisaea apoda) as possible. Still later, during an excursion along the Maros River in Celebes, he got six good specimens of Papilio androcles, one of the largest swallowtail butterflies, with long white tails dangling down like streamers. And from the island of Waigiou, just offshore from New Guinea's Vogelkop, he harvested 24 individuals of the red bird of paradise (Paradisaea rubra). His purpose in collecting multiples was not just to aggrandize supplies of the most decorative species for sale; it was also the desire to represent each species in his personal collection with a "good series" of individuals.
The consequence of such redundant collecting was that Wallace saw and recognized—to a degree that Charles Darwin had been slower to see and recognize—something momentous about creatures in the wild: That each species encompasses considerable variation among individuals. Not every specimen of Papilio androcles has tails as long and as white as every other. Not every greater bird of paradise is as great as every other. Individuals vary genetically from their siblings and cousins in ways that may manifest as visible and physiological inequities.
This insight is crucial to the idea of evolution by natural selection. Individual variation provides the differential material upon which selection works. Darwin appreciated such variation in domesticated species but became aware of its prevalence in the wild only during his long project on the classification of barnacles, an eight-year detour along his slow course toward publishing his theory. Wallace got there by a shorter route because, being forced to pay his way as a commercial collector, he constantly saw variation in his inventory.