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great phylum of molluscs, through defects in general organization;
then the insects, through their limited size. Only the vertebrates
remained. The cold-blooded forms were eliminated by the biological
invention of temperature-regulation; the birds, by their over-special-
ization for flight; the marsupials, by their greatly inferior reproduct-
ive mechanism. Among the placentals, now sole repositories of
potential advance, the majority of lines cut themselves off from pro-
gress by one-sided specialization. Only the arboreal primates escaped,
since their mode of life left teeth and limbs unspecialized, while
demanding greater efficiency in the highest sense of all, vision, and
greater correlation between hand and eye. This correlation meant
improvement in brain structure, which spilled over in the form of
increased educability and awareness. Finally, all the primate lines
but one wandered into blind alleys, becoming over-specialized for
tree life. Only the one stock which early redescended to the ground
and concentrated on all-round adaptability remained potentially
progressive — man. The human species has now become the
only branch of life in which and by which further substantial
evolutionary progress can possibly be realized. And it has achieved
this enviable, but at the same time intensely responsible, position
solely by concentrating on brain as against other organs as its line
of specialization.
This evolution of brain, as the one inexhaustible or at least un-
exhausted source of progress, thus demands our closest attention as a
biological analogy for social affairs. With some simplification, the
process of brain evolution in vertebrates is resolvable into two main
steps—first, the addition of two centres of correlation in different parts
of the brain, one for the correlation of sensory knowledge, the other
for the correlation of action, and of course with the two centres united
by communicating cables. This is the stage arrived at in fish. The
next step was the provision of a further quite new centre of correlation,
superimposed on the previous mechanism. This organ of ultimate
adjustment and control consists of the cerebral hemispheres, which are
wholly unrepresented in the lowest vertebrates. Its essential exchange
mechanism consists of the cerebral cortex. So far as we know, the
cortex, in spite of all localizations and functional specializations
within it, always acts as a whole, in the sense that its activity can be
thought of as a complex field which is altered in its total functioning
by any alteration in any of its parts.
The final step between ape and man is marked by the great enlarge-
ment of those areas of the brain which have the least specialized func-
tion—the so-called association areas, which lie between the regions