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.  The internal skeleton had to become far more highly developed before its great advantages, and freedom from disadvantages, became apparent.  The mollusk and crab were working a mine rich in surface deposits although soon exhausted.  The vertebrate lead was poor at the surface, and only later showed its inexhaustible richness.  It looked as if the vertebrate were making a very poor speculation.

Whether this explanation be true or not, a glance at a chart, showing the geological succession of occurrence of the different kingdoms, proves that in the oldest palaeozoic periods there were well-developed cuttlefish and crabs before there were any vertebrates worthy of the name.  If any were present, their skeleton was purely cartilaginous and not preserved.

I think we may go farther, although in this latter consideration we may very possibly be mistaken.  We have already seen that the progress made by any animal may be measured more or less accurately by the length of time during which its ancestors maintained a swimming life.  The ancestors of the coelenterates settled to the bottom first.  Then successively those of flatworms, mollusks, annelids, and crabs.  All this time the ancestors of vertebrates were swimming in the water above.  Food was probably more abundant, certainly more easily and economically obtained by a creeping life, on the bottom.  But thither the vertebrate could not go.  There his mail-clad competitors were too strong for him.  Those which settled and tried to compete in this sort of life perished.  We may have to except the ascidia, but they paid for their success by the loss of nearly all their vertebrate characteristics.  The future progress of vertebrates depended upon their continual activity in the swimming life.  And they were forced by their environment to maintain this.  Otherwise they might, probably would, never have attained their present height of organization.  Certainly at this time you would have found it hard to believe that the victory was to fall to these weaker and smaller vertebrates.

Let us come down to a later period.  Reptiles, mammals, and birds are struggling for supremacy.  Of the power and diversity of form of these old reptiles we have generally no adequate conception.  The forms now living are but feeble remnants.  There were huge sea-serpents, and forms like our present crocodiles, but far more powerful.  Others apparently resembled in form and habit the herbivorous and carnivorous mammals of to-day.  Others strode or leaped on two legs.  And still others flew like bats or birds.  They were terrible forms, with coats of mail and powerful jaws and teeth.  And they were active and swift.  When we look at them we see that the vertebrate, though slow in gaining the lead, is sure to hold it.  The internal skeleton gave fewer advantages at the start; its greatest superiority had lain in future possibilities.