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product of change in general size. It is what Darwin called a
"correlated character"—something useless in itself but correlated
with some other character which is useful. We now know of a great
many such correlated characters—for instance, tuning up or clown
the activity of one or other of the ductless glands to adjust the animal
to its particular environment may produce changes in colour or in
bodily proportions—and without question a great many apparently
meaningless differences characterizing related species or sub-species
are mere external signs of such invisible but insignificant inner adap-
Another old objection to Darwinian explanations of evolution is
the incredible complexity of the detailed adjustments needed to effect
a change such as the lengthening of an animal's neck. To take but
this one example: all the tendons tying the neck vertebrae together
must be strengthened and their direction adjusted. How could ran-
dom variation and selection account for this? We now know that
the tissue of which tendons are made, like many other tissues of the
body, has the faculty of responding to demands upon it—by ex-
cess growth and by changes in the direction of its fibres. Granted
this one basic adaptation, all the rest follow. The myriad detailed
adjustments are not determined by heredity and selection, but are
built anew in each individual during its development.
In these and many other ways our modern knowledge of growth
and development has lightened the burden on natural selection, at
the same time that advances in heredity have shown natural selection
to be a much more flexible instrument than the last generation of
biologists thought possible.
To sum up, Darwinism to-day is very much alive. In certain re-
spects, indeed, modern evolutionary theory is more Darwinian than
Darwin was himself. Darwin's special contribution to the evolution
problem was the theory of natural selection, but, owing to the rudi-
mentary state of knowledge in certain biological fields, he was forced
to bolster this up with subsidiary Lamarckian hypotheses, of the in-
heritance of the effects of use and disuse and of modifications produced
by the direct agency of the environment. To-day we arc able to
reject these subsidiary hypotheses, and can demonstrate that natural
selection is omnipresent and virtually the only guiding agency in
Darwin has with some justice been called the Newton of biology.
Like Newton, he gave his science a unifying concept, and one capable
of extension into every corner of its field. There are evolutionary
implications in every branch of biology. The human physiologist