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1. The process of developmental selection is a normal event or succession of events in the life cycle of vascular plants, where it assumes various forms, being represented chiefly by embryonic selection, gametophytic selection, and gametic selection. 2. Developmental selection differs materially from natural selection, germinal selection, the intraselection of Roux, as well as the other selection theories. 3. Records of conspicuous cases of polyembryony in ferns are brought together. Original studies are added, constituting definite evidence that a selective plurality of embryos may normally exist even in the leptosporangiate ferns. Nearly all living ferns seem to have embryonic selection, or show evidence of having passed through a stage in which embryonic selection was the normal condition. The embryonic selection represented by the polyembryony of gymnosperms was derived from an embryonic selection habit in their fern ancestors. 4. Developmental selection in gymnosperms and angiosperms is not only represented by a selection among embryos, but also by a selection between female gametophytes and the male gametophytes represented by the pollen tubes. 5. A form of selection intermediate between natural selection and developmental selection may be recognized in the competition between buds and branches of a sporophyte or a branching thallus. 6. Developmental selection is a process which brings into play a definite internal competition between embryonic diploid individuals, as well as between the haploid sperms of fern plants, and the haploid male and female gametophytes of gymnosperms and angiosperms. On the other hand, natural selection usually acts on the diploid generation in these plant groups, or on the haploid fern gametophytes, where selection may take place in the external environment. 7. The discussion seeks to show why the process of developmental selection is not open to the more serious objections which have been urged against natural selection, and on what basis it equals or excels the latter as an effective selective process. 8. The discussion also shows how developmental selection may account for some of the phenomena of orthogenesis on a mechanical basis. 9. Developmental selection is not responsible for the origin of the chromosomal or other intracellular phenomena involved in mutation, but it is a powerful mechanism whose censorship may determine whether or not any particular intracellular phenomena causing mutation may complete the life cycle to be heritable, and therefore recognizable as a mutation.

Variations in Cytology and Gross Morphology of Taraxacum. I. Cytology of Taraxacum laevigatum