(7.) _The natural system of organisms_, the natural grouping of all the different forms of Animals, Plants, and Protista into numerous smaller or larger groups, arranged beside and above one another; the kindred connection of species, genera, families, orders, classes, tribes, etc., more especially, however, the _arboriform branching character of the natural system_, which is the spontaneous result of a natural arrangement and classification of all these graduated groups or categories. The result attained in attempting to exhibit the relationships of the mere forms of organisms by a tabular classification is only explicable when regarded as the expression of their actual _blood relationship_; _the tree shape of the natural system_ can only be understood as the actual _pedigree of the organisms_.
(8.) _The chorology of organisms_, the science of the local distribution of organic species, of their _geographical_ and _topographical dispersion over the surface of the earth_, over the heights of mountains and in the depths of the ocean, but especially the important phenomenon that every species of organism proceeds from a so-called “_centre of creation_” (more correctly a “_primæval home_” or “_centre of distribution_”); that is, from a single locality, where it originated but once, and whence it spread.
(9.) _The œcology of organisms_, the knowledge of the sum of the _relations of organisms to the surrounding outer world_, to organic and inorganic conditions of existence; the so-called “_economy of nature_,” the correlations between all organisms living together in one and the same locality, their adaptation to their surroundings, their modification in the struggle for existence, especially the circumstances of parasitism, etc. It is just these phenomena in “the economy of nature” which the unscientific, on a superficial consideration, are wont to regard as the wise arrangements of a Creator acting for a definite purpose, but which on a more attentive examination show themselves to be the necessary results of mechanical causes.
(10.) _The unity of Biology as a whole_, the deep inner connection existing between all the phenomena named and all the other phenomena belonging to zoology, protistics, and botany, and which are simply and naturally explained by a single common principle. This principle can be no other than the common derivation of all the specifically different organisms from a single, or from several absolutely simple, primary forms like the Monera, which possess no organs. The Theory of Descent, by assuming this common derivation, throws a clear light upon these individual series of phenomena, as well as upon their totality, without which their deeper causal connection would remain completely incomprehensible to us. The opponents of the Theory of Descent can in no way explain any single one of these series of phenomena or their deeper connection with one another. So long as they are unable to do this, _the Theory of Descent remains the one adequate biological theory_.