It is of fundamental importance to determine if and how hierarchical clustering is involved in large-scale structure formation of the universe. Hierarchical evolution is characterized by rules which specify how dark matter halos are formed by the merging of halos at smaller scales. We show that scale-scale correlations of the matter density field are direct and sensitive measures to quantify this merging tree. Such correlations are most conveniently determined from discrete wavelet transforms. Analyzing two samples of Ly-alpha forests of QSOs absorption spectra, we find significant scale-scale correlations whose dependence is typical for a branching process. Therefore, models which predict a history independent evolution are ruled out and the halos hosting the Ly-alpha clouds must have gone through a history dependent merging process during their formation.
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