The origin of language (spoken and signed, as well as language-related technological systems such as writing), its relationship with human evolution, and its consequences have been subjects of study for centuries. Scholars wishing to study the origins of language must draw inferences from evidence such as the fossil record, archaeological evidence, contemporary language diversity, studies of language acquisition, and comparisons between human language and systems of communication existing among animals (particularly other primates). Many argue that the origins of language probably relate closely to the origins of modern human behavior, but there is little agreement about the facts and implications of this connection. The shortage of direct, empirical evidence has caused many scholars to regard the entire topic as unsuitable for serious study; in 1866, the Linguistic Society of Paris banned any existing or future debates on the subject, a prohibition which remained influential across much of the Western world until late in the twentieth century. Various hypotheses have been developed about how, why, when, and where language might have emerged. Still, little more has been universally agreed upon by 1996 than over a century and a half ago, when Charles Darwin's theory of evolution by natural selection had provoked a surge of speculation on the topic. Since the early 1990s, however, a number of linguists, archaeologists, psychologists, anthropologists, and others have attempted to address this issue with new, modern methods. The origin of language can be sub-divided according to some underlying assumptions: "Continuity theories" build on the idea that language exhibits so much complexity that one cannot imagine it simply appearing from nothing in its final form; therefore it must have evolved from earlier pre-linguistic systems among humans' primate ancestors. "Discontinuity theories" take the opposite approach—that language, as a unique trait which cannot be compared to anything found among non-humans, must have appeared fairly suddenly during the course of human evolution.