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THE POPULAR SCIENCE MONTHLY
is more than divergence because it implies evolution in every direction from a central form. The idea of radii, or radiations from a central form greatly assists the imagination, because a distinctive feature of paleontology is that we are constantly dealing with fragments of history. The radiations which have been discovered must be supplemented by those which remain to be discovered, and it is very remarkable how in group after group of animals these missing "radii" have turned up.
Radiation actually begins in certain single organs, and the first principle to be observed, as shown in the accompanying diagram, is
Main Lines of Adaptive Radiation of (a) limbs and feet, (b) teeth among mammals.
