orms more generalized should be placed
lower down on the ancestral tree, and must have had an earlier
geological occurrence because they represented more nearly the
ancestors of the higher. But this explains also the facts of
embryonic development.

According to Mr. Darwin's theory all the species of higher animals
have developed from unicellular ancestors. It had long been known
that all higher forms start in life as single cells, egg and
spermatozoon. And these, fused in the process of fertilization, form
still a single cell. And when this single cell proceeds through
successive embryonic stages to develop into an adult individual it
naturally, through force of hereditary habit, so to speak, treads
the same path which its ancestors followed from the unicellular
condition to their present point of development. Thus higher forms
should be expected to show traces of their early ancestry in their
embryonic life. Older and lower adult forms should represent
persistent embryonic stages of higher. It could not well be
otherwise.

But the path which the embryo has to follow from the egg to the
adult form is continually lengthening as life advances ever higher.
From egg to sponge is, comparatively speaking, but a step; it is a
long march from the egg to the earthworm; and the vertebrate embryo
makes a vast journey. But embryonic life is and must remain short.
Hence in higher forms the ancestral stages will often be slurred
over and very incompletely represented. And the embryo may, and
often does, shorten the path by "short-cuts" impossible to its
original ancestor. Still it will in general hold true, and may be
recognized as a law of vast importance, that any individual during
his embryonic life repeats very briefly the different stages through
which his ancestors have passed in their development since the
beginning of life. Or, briefly stated, ontogenesis, or the embryonic
development of the individual, is a brief recapitulation of
phylogenesis, or the ancestral development of the phylum or group.

The illustration and proof of this law is the work of the
embryologist. We have time to draw only one or two illustrations
from the embryonic development of birds. We have already seen that
the embryonic bird has the long tail of his reptilian ancestor. In
early embryonic life it has gill-slits leading from the pharynx to
the outside of the neck like those through which the water passes in
the respiration of fish. The Eustach