All organic phenomena belong, according to their causes, to two different classes: (1) Those belonging to one group are the results of external influences in each ontogeny and are not inherited; they represent nutrition varieties, are experimentally demonstrable, and constitute the subject matter of experimental physiology. (2) The others are inherited and again transmitted; they belong to the physiology of the idioplasm. This subject is mainly occupied with the origin of the determinants, hence with the formation of varieties and species. It is not the subject of experiment, and constitutes the phylogeny or the physiology of the formation of determinants. A sub-division of this subject is occupied with the development of the determinants already present, hence with the formation of races. It is elucidated especially by experiments in crossing and may be designated as the physiology of the development of the determinants. The morphological phenomena which find their application in taxonomy, belong exclusively to phylogeny. Their ontogenetic history does not explain their true significance; this can be known only in a phylogenetic way by comparison of one phenomenon with those phenomena from which it has arisen in the course of evolution. 23. PLANT CLASSIFICATION FROM THE STANDPOINT OF PHYLOGENY. Spontaneous generation has taken place at all times and in all places, in as far as the necessary conditions were concurrently present. (See page 47). After spontaneous generation the automatic phylogenetic evolution begins and advances constantly. Consequently the phylogenetic line rises from time to time to higher stages of organization and division of labor, but dies of old age if the automatic perfecting process ceases. The phylogenetic lines of organisms now living have therefore an unequal age; those of the most highly developed plants and animals had their origin in the earliest periods of organic life, those of the lowest organisms in the most recent periods. Hence no general genetic relation exists among lines now living; only those that are nearly related and have reached approximately equal stages of organization may be regarded as branches of the same phylogenetic stock. A phylogenetic plant system does not exist in fact, but only in figure. If genetic relation between two races is assumed, either as a reality or as a symbol, the degree of relationship is determined in a theoretically exact manner by the number and length of the phylogenetic steps which are found either between them both or between them and the common starting point, according as races belong to the same or collateral lines. The fact that two organisms belong to the same line of descent is recognized from the ontogeny of the higher including the ontogeny of the lower.