The genetic analysis o f taxonomic differences is still in its infancy; by far the greater majority o f animals and plant species are genetically unknown, though cytologists have already advanced rather farther in the comparative study o f their chromosomes, particularly in plants. However, the comparatively small number o f well-analysed cases are already enough to show us that species, and other taxonomic groups, may differ in any o f the ways genetically possible. We find a rough, but only a rough, correspondence between the magnitude o f the genetical differences and the taxonomic interval between different groups. The imperfections o f the correlation are perhaps only to be expected when we reflect that the word species covers groups which are very different in genetical status. Darlington2 lists six types o f species, which will be affected in different ways by the evolutionary mechanisms o f variation and inheritance. They are given in order o f increasing hybridity (i.e. genetical inhomogeneity) o f individuals.