(1.) _The palæontological history of the development of organisms_, the gradual appearance and the historical succession of the different species and groups of species, the empirical laws of the palæontological change of species, as furnished to us by the science of fossils, and more especially the _progressive differentiation and perfecting_ of animal and vegetable groups in the successive periods of the earth’s history. (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.