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It is reasonable to expect that the global dispersal of modern humans was influenced by habitat variation in space and time; but many simulation models average such variation into a single, homogeneous surface across which the dispersal process is modelled. We present a demographic simulation model in which rates of spatial range expansion can be modified by local habitat values. The broadscale vegetation cover of North America during the late last glacial is reconstructed and mapped at thousand-year intervals, 13,000-10,000 radiocarbon years BP. Results of the simulation of human dispersal into North America during the late last glacial are presented; output appears to match observed variation in occupancy of habitats during this period (as assessed from discard rates of diagnostic artefacts), if we assume that intrinsic population growth rates were fairly high and that local population densities varied as a function of environmental carrying capacity. Finally, a number of issues are raised relating to present limitations and possible future extensions of the simulation model.

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We present a detailed experimental and theoretical study of the optical response of suspensions of ferromagnetic nanoparticles (''ferroparticles'') in nematic liquid crystals (''ferronematics''), concentrating on the magnetic field-induced Frederiks transition. Even extremely low ferroparticle concentrations (at a volume fraction between 2 Â 10 À5 and 2 Â 10 À4 ), induce a significant additional ferronematic linear response at low magnetic field (<100 G) and a decrease in the effective magnetic Frederiks threshold. The experimental results demonstrate that our system has weak ferronematic behavior. The proposed theory takes into account the nematic diamagnetism and assumes that the effective magnetic susceptibility, induced by the nanoparticles, no longer dominates the response. The theory is in good agreement with the experimental data for the lowest concentration suspensions and predicts the main features of the more concentrated ones. The deviations observed in these cases hint at extra effects due to particle aggregation, which we have also observed directly in photographs.