¹ Since the post-Darwin period a “natural classification” has meant in biology a classification based on the hypothetical common descent of the organisms. This restriction of the meaning of the term is unjustified. The actual mode of descent has been ascertained, and can be ascertained, only for very few groups. But even granting the possibility of establishing the complete phylogenetic history of every organism, it has never been adequately proven that the degree of similarity between the organisms is always proportional to the closeness of their blood relationships. Some palaeontological data cast a grave doubt on this point.

² This statement applies, of course, only to the discontinuities caused by gene complexes (related species, not to speak about higher categories, differ from each other usually in many genes). Changes in single genes may produce variants so far removed from the original form that their characteristics do not overlap. Discontinuities of this nature may be preserved indefinitely in a panmictic population.