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.
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