The pool of the Central European flora consists of a majority of vascular plant taxa that are restricted to very base rich and calcareous soils. Ellenberg indicator values for Germany indicate that this floristic pattern is one of the potentially most powerful determinants of the richness of modern temperate plant communities. Considering the example of the forest flora, which, as the putative natural core of the species pool, exhibits the same skew, it is shown that neither the frequency of suitable soil types nor other correlated ecological factors can explain this striking pattern. Also, the ramification of higher taxa offers no indication of higher evolution speeds in calciphilous plants. As an alternative, it is hypothesized that Pleistocene range contractions have caused the extinction of more acidophilous than calciphilous species, because acid soils were much rarer when refugial areas were at their minimum. If this is correct, one of the most significant ecological patterns in the contemporary distribution of plant diversity must be regarded as a result of ecological drift imposed by a historical bottleneck.