Darwin’s emphasis on the species created a rush to search for new species, and to extreme species splitting. Taxonomists, to overcome problems of convergence in external structure, combined studies of external and internal structures, often of the genitalia. Darwin’s legacy called for a definition of what a species is. Some suggested that species definitions should involve reproductive criteria: A species is an isolated group of interbreeding populations. This definition is relevant only to local situations and to sexually reproducing groups. Another evolutionary definition: A species is a lineage evolving separately from others. This ignores the causation and maintenance of discontinuities between contemporary species, does not refer to why phyletic lines do not interbreed and raises any race to the rank of species. In practice, the classifying of species seeks discontinuities among groups. On a geographical scale, sharp discontinuities over short distances suggest strong reproductive isolation and indicate different species; gradual differences over large distances suggest one species, with interbreeding populations. Scientists of the century following Darwin argued that diagnostic definitions of species relate not to adaptive characters but to discontinuities, with no relation to specific advantages, and that they do not necessarily have survival value. Also, alongside the slow accumulation of hereditary advantages, evolution may proceed by mutations producing a completely new body plan in a single step. New species evolve by the drastic re-patterning of the gene sequences along the chromosome. The discovery of variation in chromosomes took taxonomy beyond the level of external morphology and internal anatomy, to that of chromosome numbers, which may be group-specific. Furthermore, at every taxonomic level, some proteins are highly specific. The proteins of one organism will show a stronger antibody reaction to those of a closely related organism than they will to those of a more distantly related organism. When one introduces foreign cells or particles (antigens) into an organism, highly specific protein molecules (antibodies) are produced in the immune system of that animal. The degree and rate of blood reaction offer an index of relationship; closely related blood react more powerfully and more rapidly than do distantly related blood. There is a general correlation between differences in blood reaction and broad lines of zoological classification. Serology, by detecting homologous proteins in the serum, can therefore be a taxonomic marker that contributes to classification. When classifying biodiversity by a few features, the resulting classification may not be repeatable with other features. Furthermore, natural groups may not share any single defining character, as members of a group might lose their diagnostic features. We should classify by overall similarity in many features, rather than by over-weighting a few. Phenetic systematics clusters individuals into groups by overall similarities, in many numerically encoded features. Computers determine overall similarities in quantitative, repeatable terms and an objective classification automatically emerges, that is theory-free and does not attempt to reflect evolution. Phylogenetic systematics (cladistics) differs from phenetics in that it classifies organisms by descent from a common ancestor, not by overall similarity. It reflects the evolutionary history of a group and corresponds to all descendants of an ancestral lineage. Features evolve, from ancestral to derived states. We classify only by shared derived features, not by ancestral ones. We reach the ancestral character state by consulting an additional group that is phylogenetically close, but not within the study group. Features occurring within both the study group and the outgroup are ancestral to the study group; those in the study group that are absent from its outgroup are derived.