on the dorsal surface because its branches ran to heavy muscles much
used in swimming. Later the other nerve-cords degenerated, for such
a degeneration of nerve-cords is not at all impossible or
improbable. "No thoroughfare" is often written across paths
previously followed by blood or nervous impulses, when other paths
have been found more economical or effective.

But where did the notochord come from? I do not know. It always
forms in the embryo out of the entoderm or layer which becomes the
lining of the intestine. Now this is a very peculiar origin for
cartilage, and the notochord is a very strange cartilage even if we
have not made a mistake in calling it cartilage at all. My best
guess would be that it is simply a thickened portion of the upper
median surface of the intestine to keep the "balls" of digesting
nutriment or other hard particles in the intestine from "grinding"
against the nerve-cord as they are crowded along in the process of
digestion. Once started its elasticity would be a great aid in
swimming.

Professor Brooks has called attention to the fact that the higher a
group stands in development, the longer its ancestors have
maintained a swimming life. Thus we have noticed that the sponges
were the first to settle; then a little later the mass of the
coelenterates followed their example. But the etenophora, the
nearest relatives of bilateral animals, have remained free swimming.
Then the flat worms and mollusks took to a creeping mode of life,
while the annelids and vertebrates still swam. Then the annelids
settled to the bottom and crept, and all their descendants remained
creeping forms. The vertebrates alone remained swimming, and
probably neither they nor their descendants ever crept until they