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A simplified derivation of the Kubo-Greenwood-Peierls equation is given using the quantum theory of fluctuations developed by Callen and Welton.

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We examine the distribution of the relative fractional fluctuation of the magnetisation of a 3D Ising ferromagnet at the critical temperature Tc by considering a histogram of frequency of the fractional magnetisation. The distribution is highly non-Gaussian and is reminiscent of the histogram of extinction intensity for fossil genera shown by Raup1 (1991) (see also Newman (1996)). The graph of relative fractional change ofmagnetisation vs MC time is compatible in appearance with the graph showing the variation of the number of extinction of families of marine animals with time (over hundreds of millions of years) given by Raup (1991), Sepkoski (1993) or Newman (1996). Our results show a power law dependence usually taken as evidence for the existence of a critical system.