(2.) The individual history of development of organisms, embryology and metamorphology, the gradual changes in the slow development of the body and its particular organs, especially the progressive differentiation and perfecting of the organs and parts of the body in the successive periods of the individual development.
(3.) The inner causal connection between ontogeny and phylogeny, the parallelism between the individual history of the development of organisms, and the palæontological history of the development of their ancestors, a connection which is actually established by the laws of Inheritance and Adaptation, and which may be summed up in the words: ontogeny, according to the laws of inheritance and adaptation, repeats in its large features the outlines of phylogeny.
(4.) The comparative anatomy of organisms, the proof of the essential agreement of the inner structure of kindred organisms, in spite even of the greatest difference of external form in the various species; their explanation by the causal dependence of the internal agreement of the structure on Inheritance, the external dissimilarity of the bodily form on Adaptation.
(5.) The inner causal connection between comparative anatomy and the history of development, the harmonious agreement between the laws of the gradual development, the progressive differentiation and perfecting, as they may be seen in comparative anatomy on the one hand, in ontogeny and palæontology on the other.
(6.) Dysteleology, or the theory of purposelessness, the name I have given to the science of rudimentary organs, of suppressed and degenerated, aimless and inactive, parts of the body; one of the most important and most interesting branches of comparative anatomy, which, when rightly estimated, is alone sufficient to refute the fundamental error of the teleological and dualistic conception of Nature, and to serve as the foundation of the mechanical and monistic conception of the universe.
(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.
We should, on account of the grand proofs just enumerated, have to adopt Lamarck’s Theory of Descent for the explanation of biological phenomena, even if we did not possess Darwin’s Theory of Selection. The one is so completely and directly proved by the other, and established by mechanical causes, that there remains nothing to be desired. The laws of Inheritance and Adaptation are universally acknowledged physiological facts, the former traceable to propagation, the latter to the nutrition of organisms. On the other hand, the struggle for existence is a biological fact, which with mathematical necessity follows from the general disproportion between the average number of organic individuals and the numerical excess of their germs. But as Adaptation and Inheritance in the struggle for life are in continual interaction, it inevitably follows that natural selection, which everywhere influences and continually changes organic species, must, by making use of divergence of character, produce new species. Its influence is further especially favoured by the active and passive migrations of organisms, which go on everywhere. If we give these circumstances due consideration, the continual and gradual modification or transmutation of organic species will appear as a biological process, which must, according to causal law, of necessity follow from the actual nature of organisms and their mutual correlations.