 The taxa in the classical Linnean system of biological classification are nested in a hierarchy, as required by Criterion 5 in Section 1.1.1. But, in certain respects, chemical classification fails to meet this requirement. The phenomenon of allotropy is exhibited when an element exists in two or more distinct forms. The element carbon has several, including the allotropes diamond and graphite. Tin has two allotropes at room tempreature, white tin (which is metallic) and grey tin (which is non-metallic). Some instances of tin will fall into one category and other instances will fall into the other category. Consequently, classification by element and classification by metal or non-metal cannot be combined hierarchically.