To sum up: we have seen that self-fertilization does occur in hermaphrodite animals, where otherwise the species would be in danger of extinction, but this is never the _sole_ and exclusive mode of fertilization[25], for hermaphrodite species have always the possibility of securing inter-crossing of individuals, and that in various ways, whether by the intervention of 'primordial males' or by an occasional or a periodic alternation of self-fertilization and mutual fertilization. Pure parthenogenesis enduring through innumerable generations does appear to occur, but in most cases unisexual generations alternate with bi-sexual, so that a stereotyping of the germ-plasm with complete uniformity of ids is obviated. [25] As to the cases Maupas has brought into notice, of permanent and apparently exclusive self-fertilization in Rhabditidæ (round worms), it seems fair to say that they have not been as yet sufficiently investigated to admit of a secure appreciation of their value in their theoretical bearings. Cf. _Arch. Zool. Exper._, 3rd ser., vol. viii, 1900. We must now briefly consider the higher plants with reference to the maintenance of diversity in the germ-plasm through crossing. We saw in an earlier lecture that most flowers are hermaphrodite, but that they do not fertilize themselves, and are adapted for crossing, since the pollen of one flower is carried by insects to the pistil of another, which cannot be reached by its own pollen, either because it ripens too early or too late, or because the stigma, notwithstanding its proximity, is so placed as to be out of reach of the pollen from the adjacent stamens. I showed, following the fundamental investigations of Sprengel, Charles Darwin, Hermann Müller, and other successors of Darwin, that the flowers may in a sense be regarded as the resultants of the insect-visits, since all their accessory adaptations--large coloured petals, fragrance, nectar, and even little minutiæ of colour and markings (honey-guides)--as well as their detailed shape, as seen in 'landing stages,' corolla tubes, and so on, are only intelligible when we refer their existence to natural selection. We assume that each of these adaptations secured some advantage for the species concerned, and that therefore their first beginnings as slight germinal varieties were accepted, and were brought gradually to their full expression by the united operation of germinal and personal selection. This at least is how we should express ourselves now that we have become acquainted with the factor of germinal selection. The advantage secured by every such improvement in a flower's means of attracting insects is obvious, as soon as it is established that cross-fertilization is more advantageous for the species than self-fertilization.