The incalculable importance of the light cast over the whole field of
human knowledge of nature by these results is patent to everyone. They
are destined every year increasingly to manifest their transforming
influence in all departments of knowledge, the more the conviction of
their irrefragable truth forces its way. And it is only the ignorant or
narrow-minded who can now doubt their truth. If, indeed, here and there,
one of the older naturalists still disputes, the foundation on which they
rest, or demands proofs which are wanting (as happened a few weeks ago on
the part of a famous German pathologist at the Anthropological Congress
in Moscow), he only shows by this that he has remained a stranger to the
stupendous advances of recent biology, and above all of anthropogeny. The
whole literature of modern biology, the whole of our present zoology and
botany, morphology and physiology, anthropology and psychology, are
pervaded and fertilised by the theory of descent.[14]

Just as the natural doctrine of development on a monistic basis has
cleared up and elucidated the whole field of natural phenomena in their
physical aspect, it has also modified that of the phenomena of mind,
which is inseparably connected with the other. Our human body has been
built up slowly and by degrees from a long series of vertebrate
ancestors, and this is also true of our soul; as a function of our brain
it has gradually been developed in reciprocal action and re-action with
this its bodily organ. What we briefly designate as the "human soul," is
only the sum of our feeling, willing, and thinking--the sum of those
physiological functions whose elementary organs are constituted by the
microscopic ganglion-cells of our brain. Comparative anatomy and ontogeny
show us how the wonderful structure of this last, the organ of our human
soul, has in the course of millions of years been gradually built up from
the brains of higher and lower vertebrates. Comparative psychology