Wallace is best known for his collecting trip to the Malay Archipelago that led to him producing a theory of evolution by natural selection around the same time, 1858, that Charles Darwin was writing On the Origin of Species. Wallace described how it came about in his autobiography:
"The problem then was not only how and why do species change, but how and why do they change into new and well defined species, distinguished from each other in so many ways; why and how they become so exactly adapted to distinct modes of life; and why do all the intermediate grades die out (as geology shows they have died out) and leave only clearly defined and well marked species, genera, and higher groups of animals? It then occurred to me that these causes or their equivalents are continually acting in the case of animals also; and as animals usually breed much more quickly than does mankind, the destruction every year from these causes must be enormous in order to keep down the numbers of each species, since evidently they do not increase regularly from year to year, as otherwise the world would long ago have been crowded with those that breed most quickly. Vaguely thinking over the enormous and constant destruction which this implied, it occurred to me to ask the question, why do some die and some live? And the answer was clearly, on the whole the best fitted live ... and considering the amount of individual variation that my experience as a collector had shown me to exist, then it followed that all the changes necessary for the adaptation of the species to the changing conditions would be brought about ... In this way every part of an animals organization could be modified exactly as required, and in the very process of this modification the unmodified would die out, and thus the definite characters and the clear isolation of each new species would be explained."
Politically Wallace was an eclectic reformer who described himself as a socialist, a Spiritualist who argued for women's suffrage and the nationalisation of the land as well as speaking out against militarism, eugenics and currency being based on gold or silver. I like the line in his 1890 article Human Selection where he writes, "Those who succeed in the race for wealth are by no means the best or the most intelligent."