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