II. The brain development of the lowest and earliest mammals (the monotremes, marsupials, and prochoriates) is closely allied to that of their palæozoic ancestors, the Carboniferous amphibia (the _stegocephala_) and the Permian reptiles (the _tocosauria_). III. During the Tertiary period commences the typical development of the cerebrum, which distinguishes the younger mammals so strikingly from the older. IV. The special development (quantitatively and qualitatively) of the cerebrum which is so prominent a feature in man, and which is the root of his pre-eminent psychic achievements, is only found, outside humanity, in a small section of the most highly developed mammals of the earlier Tertiary epoch, especially in the anthropoid apes. V. The differences of brain structure and psychic faculty which separate man from the anthropoid ape are slighter than the corresponding interval between the anthropoid apes and the lower primates (the earliest simiæ and prosimiæ). VI. Consequently, the historical, gradual evolution of the human soul from a long chain of higher and lower mammal souls must, by application of the universally valid phyletic laws of the theory of descent, be regarded as a _fact_ which has been scientifically proved. CHAPTER X CONSCIOUSNESS Consciousness as a Natural Phenomenon--Its Definition--Difficulties of the Problem--Its Relation to the Life of the Soul--Our Human Consciousness--Various Theories: I. Anthropistic Theory (Descartes); II. Neurological Theory (Darwin); III. Animal Theory (Schopenhauer); IV. Biological Theory (Fechner); V. Cellular Theory (Fritz Schultze); VI. Atomistic Theory--Monistic and Dualistic Theories--Transcendental Character of Consciousness--The Ignorabimus Verdict of Du Bois-Reymond--Physiology of Consciousness--Discovery of the Organs of Thought by Flechsig--Pathology--Double and Intermittent Consciousness--Ontogeny of Consciousness: Modifications at Different Ages--Phylogeny of Consciousness--Formation of Concepts