The differences referred to fall into the categories of either practice or doctrine, both of them presented as the intent of the relevant scriptures. In the case of religious customs governing personal life, it is helpful to view the subject against the background of comparable features of material life. It is most unlikely that diversity in hygiene, dress, medicine, diet, transportation, warfare, construction or economic activity, however striking, would any longer be seriously advanced in support of a theory that humanity does not in fact constitute one people, single and unique. Until the opening of the twentieth century, such simplistic arguments were commonplace, but historical and anthropological research now provides a seamless panorama of the process of cultural evolution by which these and countless other expressions of human creativity came into existence, were transmitted through successive generations, underwent gradual metamorphoses and often spread to enrich the lives of peoples in far distant lands. That present-day societies represent a wide spectrum of such phenomena, therefore, does not in any way define a fixed and immutable identity of the peoples concerned, but merely distinguishes the stage through which given groups are—or at least until recently have been—passing. Even so, all such cultural expressions are now in a state of fluidity in consequence of the pressures of planetary integration.