Age-long Procession of Life
Just in the same way we may marshal the countless species of
animals and plants to-day in such order that they will, in a
general way, exhibit to us the Age-long Procession of Life . From
the very start of living evolution certain forms dropped out of
the onward march, and have remained, to our great instruction,
what their ancestors were millions of years ago. People create a
difficulty for themselves by imagining that, if evolution is
true, all animals must evolve. A glance at our own fellows will
show the error of this. Of one family of human beings, as a
French writer has said, one only becomes a Napoleon; the others
remain Lucien, Jerome, or Joseph. Of one family of animals or
trees, some advance in one or other direction; some remain at the
original level. There is no "law of progress." The accidents of
the world and hereditary endowment impel some onward, and do not
impel others. Hence at nearly every great stage in the upward
procession through the ages some regiment of plants or animals
has dropped out, and it represents to-day the stage of life at
which it ceased to progress. In other words, when we survey the
line of the hundreds of thousands of species which we find in
nature to-day, we can trace, amid their countless variations and
branches, the line of organic evolution in the past; just as we
could, from actual instances, study the evolution of a British
house, from the prehistoric remains in Devonshire to a mansion in
Park Lane or a provincial castle.
Another method of retracing the lost early chapters in the
development of life is furnished by embryology. The value of this
method is not recognised by all embryologists, but there are now
few authorities who question the substantial correctness of it,
and we shall, as we proceed, see some remarkable applications of
it. In brief, it is generally admitted that an animal or plant is
apt to reproduce, during its embryonic development, some of the
stages of its ancestry in past time. This does not mean that a
higher animal, whose ancestors were at one time worms, at another
time fishes, and at a later time reptiles, will successively take
the form of a little worm, a little fish, and a little reptile.
The embryonic life itself has been subject to evolution, and this
reproduction of ancestral forms has been proportionately
disturbed. Still, we shall find that animals will tend, in their
embryonic development, to reproduce various structural features
which can only be understood as reminiscences of ancestral
organs. In the lower animals the reproduction is much less
disturbed than in the higher, but even in the case of man this
law is most strikingly verified. We shall find it useful
sometimes at least in confirming our conclusions as to the
ancestry of a particular group.
We have, therefore, two important clues to the missing chapters
in the story of evolution. Just as the scheme of the evolution of
worlds is written broadly across the face of the heavens to-day,
so the scheme of the evolution of life is written on the face of
living nature; and it is written again, in blurred and broken
characters, in the embryonic development of each individual. With
these aids we set out to restore the lost beginning of the epic
of organic evolution.
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