Since only a proportionately small number of known forms can appear as types of the supposed stages of evolution, only a few phylogenetic lines, and these only in a general way, may be established, on account of the great incompleteness of the present plant world. Such a line proceeds from the green filamentous algae through the liverworts to the vascular plants. Among the phanerogams, apparently so numerously represented, only phylogenetic series of individual organs can be ascertained, but no phylogenetic series of families. A phylogenetic system of phanerogams is not to be hazarded in the roughest outline. Even the relative rank of the two chief divisions of the angiosperms, the monocotyledons and dicotyledons, is a matter of question, as also which family in each of these divisions is to be considered the most perfect. APPENDIX. TRANSLATORS' NOTES. _The Mechanico-physiological Theory of Evolution_, (_Mechanisch-Physiologische Theorie der Abstammungslehre_), by Carl von Naegeli, was published in Munich and Leipsic in 1884 in a large octavo volume of 822 pages, including two large appendices. The _Abstammungslehre_ proper, including the summary, occupies 552 pages, and constitutes, in its way, one of the most important contributions to theoretical biology. It is difficult to understand how a work of so much consequence should have received such comparatively small notice in this country, especially as Naegeli's theories seemed calculated by nature to appeal much more strongly to American students than do, for instance, those of Weismann, who has been studied ten times as much as Naegeli. This is doubtless due, in part, to the fact that we have had no English translation of Naegeli's work, a circumstance much to be regretted. The foregoing translation of the summary from _Abstammungslehre_ goes but a small way toward making Naegeli's theories accessible to English-reading students, but it will, at least, be better than nothing. The work covers a great range of subjects, all, however, having a certain relationship to each other. In the main part of the book the discussion is presented in the following order: (1) Idioplasm as bearer of the inheritable determinants; (2) Spontaneous generation; (3) Causes of variation; (4) Determinants and visible characters, in which the origin and function of the determinants is presented; (5) Variety, race, "nutrition variety," heredity and variation; (6) Criticism of the Darwinian theory of natural selection, in which the author urges seven objections to that theory; (7) Laws of evolution of the plant kingdom; (8) Alternation of generations from the standpoint of phylogeny; (9) Morphology and classification as phylogenetic sciences; (10) A comprehensive summary of the whole work, a translation of which is given in the foregoing pages.