If we examine this parallelism of the three organic series of development more accurately, we have to add the following special qualifications. _Ontogeny_, or the history of the individual development of every organism (embryology and metamorphology), presents us with a simple _unbranching_ or graduated chain of forms; and so it is with that _portion of phylogeny_ which comprises the palæontological history of development of the _direct ancestors only_ of an individual organism. But _the whole of phylogeny_—which meets us in the _natural system_ of every organic tribe or phylum, and which is concerned with the investigation of the palæontological development _of all_ the branches of this tribe—forms a _branching_ or tree-shaped developmental series, a veritable pedigree. If we examine and compare the branches of this pedigree, and place them together according to the degree of their differentiation and perfection, we obtain the tree-shaped, branching, _systematic developmental series of comparative anatomy_. Strictly speaking, therefore, the latter is parallel to _the whole of phylogeny_, and consequently is only partially parallel to ontogeny; for ontogeny itself is parallel only to _a portion_ of phylogeny. All the phenomena of organic development above discussed, especially the threefold genealogical parallelism, and the laws of differentiation and progress, which are evident in each of these three series of organic development, and, further, the whole history of rudimentary organs, are exceedingly important proofs of the truth of the Theory of Descent. For by it alone can they be explained, whereas its opponents cannot even offer a shadow of an explanation of them. Without the Doctrine of Filiation, the fact of organic development in general cannot be understood. We should therefore, for this reason alone, be forced to accept Lamarck’s Theory of Descent, even if we did not possess Darwin’s Theory of Selection. CHAPTER XIII. THEORY OF THE DEVELOPMENT OF THE UNIVERSE AND OF THE EARTH. SPONTANEOUS GENERATION. THE CARBON THEORY. THE PLASTID THEORY.