unmistakable verdict of history, and we shall do well to heed it.

But granting all that can be claimed for this sequence, have not the
lower forms whose anatomy we have sketched--worm, fish, and
bird--halted at various points along this line of march? Yet they
have evidently survived. And if they have found safe resting-places,
cannot higher forms turn back and join them? In other words, is not
degeneration easier than advance and just as safe? What is the
result if an animal tries to return to a lower plane of life or
refuses to take the next upward step? Generally extermination. The
very classification of worms in a number of small isolated groups,
which must once have been connected by a host of intermediate forms,
is indisputable proof of most terrible extermination. They did not
go forward, and the survivors are but an infinitesimal fraction of
those which perished. Let us take an illustration where palaeontology
can help us. The earth was at one time covered with marsupial
mammals. Some advanced into placental forms. The great mass remained
behind. And outside of Australia the opossums are the only survivors
of them all. And this is only one example where a thousand could be
given. Place is not long reserved for mere cumberers of the ground.
There are so few exceptions to this statement that we might almost
call it a law of biology.

Let us see how it fares with an animal which retreats to a lower
plane of life. A worm, rather than seek its own food, becomes a
parasite. It degenerates, but still is easily recognized as a worm.
A crustacean tries the same experiment, though living outside of its
host instead of in it. It sinks to a place even lower, if possible,
than that of the parasitic worm. A locomotive form becomes sessile.
It loses most of its muscles and the larger part of its nervous
system; and even the digestive system, which it has made the goal of
its existence, is inferior to that of its locomotive ancestors and
relatives. But to the vertebrate these lowest depths of stagnation
and degeneration are, as a rule, impossible. From true fish upward
parasitism and sessile life are practically impossible. Here
stagnation and degeneration mean, as a rule, extinction. Of all the
relatives of vertebrates back to worms only the very aberrant lines
of amphioxus and of the tunicata remain. Of the rest not a single
survivor has yet been discovered. And yet what hosts of species must
have peopled the sea. The pr