and motion,--can be shown to exist in all multicellular organisms as
functions of the cells of which their bodies are composed. In the lowest
Metazoa, the invertebrate sponges and polyps, there are, just as in
plants, no special soul-organs developed, and all the cells of the body
participate more or less in the "soul-life." It is only in the higher
animals that the soul-life is found to be localised and connected with
special organs. As a consequence of division of labour, there have here
been developed various sense-organs as organs of specific sensibility,
muscles as organs of motion and volition, nerve-centres or ganglia as
central co-ordinating and regulating organs. In the most highly developed
families of the animal kingdom, these last come more and more into the
foreground as independent soul-organs. In correspondence with the
extraordinarily complicated structure of their central nervous system
(the brain with its wonderful complex of ganglion-cells and
nerve-fibres), the many-sided activity of such animals attains a
wonderful degree of development.

It is only in these most highly-developed groups of the animal kingdom
that we can with certainty establish the existence of those most perfect
operations of the central nervous system, which we designate as
consciousness. As we know, it is precisely this highest brain-function
that still continues to be looked upon as a completely enigmatical
phenomenon, and as the best proof for the immaterial existence of an
immortal soul. It is usual at the same time to appeal to Du
Bois-Reymond's well-known "Ignorabimus address on the Boundaries of
Natural Knowledge" (1872). It was by a peculiar irony of fate that the
famous lecturer of the Berlin Academy of Science, in this much-discussed
address of twenty years ago, should be representing consciousness as an
incomprehensible marvel, and as presenting an insuperable barrier to
further advances of knowledge, at the very moment that David Friedrich