 Prior tradition had been heavily affected by Buffon’s novel conception of biological species in which he made a sharp distinction between “natural” species defined by such properties as fertile interbreeding, and “artificial” species and varieties defined by morphological traits and measurements upon these (see the entry on  evolutionary thought before Darwin,  Section 3.3). This distinction was utilized selectively by Darwin. Particularly as conceptualized by German natural historians of the early nineteenth-century, “Buffonian” species were defined by the unity of common descent, and distinguished by their historical and ontological character from the taxonomic species of Linnaean natural history. This distinction between “natural” and “logical” species had maintained a distinction between problems of practical classification of preserved specimens, and those relating to the unity of natural species, which most maintained as fixed on the basis of reproductive unity and the sterility criterion (Sloan 2008).