Let us go back to two forms having much the same grade of
organization: both worms. One of them sets out to become a
vertebrate, building an internal skeleton. The other forms an
external skeleton and becomes a crab. To form its skeleton the crab
had only to thicken the cuticle already present in the annelid. It
had to modify the already existing parapodia and their muscles,
changing them to legs. The external skeleton gave from the start a
double advantage--protection and better locomotion. Every grain of
thickening aided the animal in the struggle for existence in both
these ways. The very fact that the skeleton was external may have
rendered it more liable to variation, because it was thus exposed to
continual stimuli. And the best were rapidly sifted out by Natural
Selection. The change and development went on with comparative
rapidity. In the mollusk the change was apparently still more easy
and the development still more rapid.
But the development of an internal skeleton was more difficult and
slower. It was of no use for the protection of the animal, and only
gradually did it become of much service in locomotion. Being
deep-seated it very possibly changed all the more slowly.
Furthermore, a cartilaginous rod, like the notochord, even fully
developed, hardly enabled the animal to fight directly with the
mail-clad crab.
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