Abstract

Charles Darwin was one of the first to propose a unified framework with which to understand human and animal behavior. The foundation of Darwin’s framework is his theory of descent with modification. What Darwin was convinced that theory allowed him to say about human and animal behavior is exemplified in the ‘continuity thesis.’ As Darwin put it, ‘there is a much wider interval in mental power between one of the lowest fishes, as a lamprey or lancelet, and one of the higher apes, than between an ape and a man; yet this interval is filled up by numberless gradations’ (Darwin 1871 [1936]: 453). Darwin’s continuity thesis is the foundation of contemporary studies of animal behavior; it is, along with contemporary evolutionary theory, what unifies the field of animal behavior.