I am well aware that some very eminent systematists regard the whole problem as solved. They hold as Darwin did that specific diversity has no physiological foundation or causation apart from fitness, and that species are impermanent groups, the delimitations of which are ultimately determined by environmental exigency or "fitness." The specific diversity of living things is thus regarded as being something quite different in nature from the specific diversity of inorganic substances. In practice those who share these opinions are, as might be anticipated, to be found among the 'lumpers' rather than among the 'splitters.' In their work, certainly, the Darwinian theory is actually followed as a guiding principle; unanalysed inter-gradations of all kinds are accepted as impugning the integrity of species; the underlying physiological problem is forgotten, and while the product is almost valueless as a contribution to biological research, I can scarcely suppose that it aids greatly in the advances of other branches of our science. But why is it that, with these exceptions, the consequences of the admittedly general acceptance of a theory of evolution are so little reflected in the systematic treatment of living things? Surely the reason is that though the systematist may be convinced of the general truth of the evolution theory at large, he is still of opinion that species are really distinct things. For him there are still 'good' species and 'bad' species and his experience tells him that the distinction between the two is not simply a question of degree or a matter of opinion.