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Although the authors went on to show evidence that rates of nucleotide substitution are not constant across vertebrates \cite{LangleyFitch1974}, they maintained that comparison of their phylogenetic dating of mammals against geological dates showed a remarkably good fit.   Evidence for or against the existence of a molecular clock (sometimes known as the `rate-constancy hypothesis') provided a key battleground on which fueled debate between the  neutralist and selectionist views of molecular evolutionwere contested  \cite{Kimura1987}. Kimura \cite{Kimura1987} put it thus: