Even the numerous experiments of that admirable German botanist, C. F. Gärtner, although they advanced matters further, did not suffice to make the relations between insects and flowers thoroughly clear; for this the basis of the theory of Descent and Selection was necessary. Here, again, it was reserved for Charles Darwin to lead the way where both contemporaries and predecessors had been blindly groping. He recognized that, _in general_, self-fertilization is disadvantageous to plants; that they produce fewer seeds, and that these produce feebler plants, than when they are cross-fertilized; that, therefore, those flowers which are arranged to secure cross-fertilization have an advantage over those which are self-fertilized. In many species, as Sprengel had already pointed out, self-fertilization leads to actual infertility; only a few plants are as fertile with their own pollen as with that of another plant; and Darwin believed that, in all flowering plants, crossing with others of the same kind, at least from time to time, is necessary if they are not to degenerate. Thus the advantage which the flowers derive from the visits of insects lies in the fact that insects are instrumental in the cross-fertilization of the flowers, and we can now understand how the plant was able to vary in a manner favourable to the insect-visits, and to exhibit adaptations which serve exclusively to make these visits easier; we understand how it was possible that there should develop among flowers an endless number of contrivances which served solely to attract insects, and even how, for the same end, the insignificant blossoms of the oldest Phanerogams must have been transformed into real flowers. We must not imagine, however, that the obviously important crossing of plant-individuals, usually called 'cross-pollination,' can be effected only by means of insects. There were numerous plants in earlier times, and there is still a whole series in which cross-fertilization is effected through the air by the wind; these are the anemophilous or wind-pollinated Angiosperms. To these belong most of the catkin-bearers, such as hazel and birch, and also the grasses and sedges, the hemp and the hop, and so forth. In these plants there is no real flower, but only an inconspicuous blossom, without brightly-coloured outer envelopes, without fragrance or nectar; all of them have smooth pollen grains, which easily separate into fine dust and are carried away by the wind until they fall, by chance, far from their place of origin, on the stigma of a female blossom. [Illustration: FIG. 41. Flower of Meadow Sage (_Salvia pratensis_), after H. Müller. _st´_, immature anthers concealed in the 'helmet' of the flower. _st´´_, mature anther lowered. _gr´_, immature stigma. _gr´´_, mature stigma. _U_, the lower lip of the corolla, the landing-stage for the bee.]