Written partly in response to Social Darwinism and in particular to Thomas H. Huxley's Nineteenth Century essay, "The Struggle for Existence", Kropotkin's book drew on his experiences in scientific expeditions in Siberia to illustrate the phenomenon of cooperation. After examining the evidence of cooperation in nonhuman animals, pre-feudal societies, in medieval cities, and in modern times, he concludes that cooperation and mutual aid are the most important factors in the evolution of the species.