We have discussed this already; we saw that experiments instituted by Darwin proved that seedlings which had arisen through cross-fertilization were superior to those arising through self-fertilization, and that in many cases the mother-plant itself produced fewer seeds when self-fertilized than when cross-fertilized. This discovery afforded an explanation of the cross-fertilization of flowers by insects which Sprengel had previously observed. We understand how the flowers must have become so adapted through processes of selection that they were unable to fertilize themselves, but attracted insects, and, so to speak, compelled these to dust them with pollen from another plant of the same species. We also understand how self-fertilization remained possible for many flowers in the event of cross-fertilization through insects not being effected, since after a certain period of waiting, a curvature of the stamens or the pistil may take place and lead to the stigma being dusted with the pollen of the same flower. Obviously the development of _fewer_ seeds is preferable to complete sterility. It is a well-known fact that peculiar inconspicuous and closed flowers, designed solely for self-fertilization, may occur along with the open flowers, as in the case of the so-called cleistogamous flowers of the violet (_Viola_) and the little dead-nettle (_Lamium amplexicaule_), and the phyletic origin of these becomes intelligible as soon as it is established that cross-fertilization is more advantageous than self-fertilization.