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.
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