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usually called specializations arc one-sided. This distinction contains
the kernel of what is probably the most important of our biological
analogies—the analogy concerning desirable and undesirable direc-
tions of change.
A detailed analysis of type of evolutionary change shows that some
of them can legitimately be called progressive, in the sense thai they
constitute part of a steady advance on the part of living matter toward
a greater control over and independence of its environment. Only a
small and steadily diminishing fraction of life participates in pro-
gressive change.
Each step in progress is constituted by all-round spee.iali/alion—
an improvement in general organization ; one-sided specialization
always leads into an evolutionary blind alley.
Here I have only space to mention the two types of change which
have been most important in the later phases of evolutionary progress.
One is the development of mechanisms for regulating the internal
environment of an animal, and so making it more largely independent
of changes in external environment or better able to pass from one
type of activity to another. The other is the improvement of the
mechanisms for obtaining and utilizing knowledge* of the environ-
ment, which in its later stages, after the dlincncy of sense-organs had
reached its limit, has been brought about by improvement in brain
The biological analogy from the former is obvious. It provides the
most abundant justification for the abandonment of lai&wr-fairc, in
favour of social and economic planning: but the planning must be
designed to give society an internal environment which shall be both
stable in essentials and flexible in detail, and to enable it to undertake
the greatest diversity of functions with the least dislocation.
The biological analogy from brain evolution is, however, oven more
illuminating. As animal evolution continued, the avenues of progress
were cut off one by one. Changes that had been progressive in their
time were exploited to the full and reached the limit of their potential-
ities. Mere bulk of body had reached its limit in the dinosaurs during
the Mesozoic, some sixty million years ago. Ten or twenty million
years later, temperature-regulation in certain animal forms had been
perfected. The exploitation of the insectan type of social life by ants
was over about twenty-five million years back, and ants have not
evolved since.
Similarly, the number of the groups which might share in pro-
gressive change steadily narrowed down. Groups like the cchino-
derms were soon eliminated owing to their headlessness; then the