In my opinion, all depends upon a right understanding of this _philosophical foundation of the Theory of Descent_ and of the _pithecoid theory_ which is inseparable from it. Many persons will probably admit this, and yet at the same time maintain that all this applies only to the _bodily_, not to the _mental_ development of man. Now, as we have hitherto been occupied only with the former, it is perhaps necessary here to cast a glance at the latter, in order to show that it is also subject to the great general law of development. In doing this it is above all necessary to recollect that body and mind can in fact never be considered as distinct, but rather that both sides of nature are inseparably connected, and stand in the closest interaction. As even Goethe has clearly expressed it—“matter can never exist and act without mind, and mind never without matter.” The artificial discord between mind and body, between force and matter, which was maintained by the erroneous dualistic and teleological philosophy of past times has been disposed of by the advances of natural science, and especially by the theory of development, and can no longer exist in face of the prevailing mechanical and monistic philosophy of our day. How human nature, and its position in regard to the rest of the universe, is to be conceived of according to the modern view, has been minutely discussed by Radenhausen in his “Isis,”(33) which is excellent and well worth perusal. With regard to the origin of the human mind or the soul of man, we, in the first place, perceive that in every human individual it develops from the beginning, step by step and gradually, just like the body. In a newly born child we see that it possesses neither an independent consciousness, nor in fact clear ideas. These arise only gradually when, by means of sensuous experience, the phenomena of the outer world affect the central nervous system. But still the little child is wanting in all those differentiated emotions of the soul which the full-grown man acquires only by the long experience of years. From this graduated development of the human soul in every single individual we can, in accordance with the inner causal connection between ontogeny and phylogeny, directly infer the gradual development of the human soul in all mankind, and further, in the whole of the vertebrate tribe. In its inseparable connection with the body, the human soul or mind has also had to pass through all those gradual stages of development, all those various degrees of differentiation and perfecting, of which the hypothetical series of human ancestors sketched in a late chapter gives an approximate representation.