I have already mentioned (p. 47) that the many vain attempts to define the idea of species theoretically have nothing whatever to do with the practical distinction of species. The extensive practical application of the idea of species, as it is carried out in systematic zoology and botany, is very instructive as furnishing an example of human folly. Hitherto, by far the majority of zoologists and botanists, in distinguishing and describing the different forms of animals and plants, have endeavoured, above all things, to distinguish accurately kindred forms as so many “good species.” However, it has been found scarcely possible, in any group, to make an accurate and consistent distinction of such “genuine or good species.” There are no two zoologists, no two botanists, who agree in all cases as to which of the nearly related forms of a genus are good species, and which are not. All authors have different views about them. In the genus _Hieracium_, for example, one of the commonest genera of European plants, no less than 300 species have been distinguished in Germany alone. The botanist Fries, however, only admits 106, Koch only 52, as “good species,” and others accept scarcely 20. The differences in the species of brambles (Rubus) are equally great. Where one botanist makes more than a hundred species, a second admits only about one half of that number, a third only five or six, or even fewer species. The birds of Germany have long been very accurately known. Bechstein, in his careful “Natural History of German Birds,” has distinguished 367 species, L. Reichenbach 379, Meyer and Wolff 406, and Brehm, a clergyman learned in ornithology, distinguishes even more than 900 different species. Thus we see that here, and, in fact, in every other domain of systematic zoology and botany, the most arbitrary proceedings prevail, and, from the nature of the case, must prevail. For it is quite impossible accurately to distinguish varieties and races from so-called “good species.” _Varieties are commencing species._ The variability or adaptability of species, under the influence of the struggle for life, necessitates the continual and progressive separation or differentiation of varieties, and the perpetual delimitation of new forms. Whenever these are maintained throughout a number of generations by inheritance, whilst the intermediate forms die out, they form independent “new species.” The origin of new species by division of labour, or separation, divergence, or differentiation of varieties, is therefore a _necessary consequence of natural selection_.(37)