Granted that each species has arisen by evolution from some
other, this germ-cell which is observed in the body of the threadworm,
must be regarded as part of what may well be called a stream of
germ-plasm, that reaches back to the beginning of life in the world. It
will be equally evident that these is no foreordained limit to the
forward extension of the stream. It will continue in some branch, as
long as there are any threadworms or descendants of threadworms in the
world.
The reader may well express doubt as to whether what has been
demonstrated for the threadworm can be demonstrated for the higher
animals, including man. It must be admitted that in many of these
animals conditions are too unfavorable, and the process of embryology
too complicated, or too difficult to observe, to permit as distinct a
demonstration of this continuity of the germ-plasm, wherever it is
sought. But it has been demonstrated in a great many animals; no facts
which impair the theory have been discovered; and biologists therefore
feel perfectly justified in generalizing and declaring the continuity of
germ-plasm to be a law of the world of living things.
Focusing attention on its application to man, one sees that the race
must represent an immense network of lines of descent, running back
through a vast number of different forms of gradually diminishing
specialization, until it comes to a point where all its threads merge in
one knot--the single cell with which it may be supposed that life on
this globe began.
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