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 Eustachian tube and the canal of the
external ear of man, separated only by the "drum," are nothing but
such an old persistent gill-slit. No gills ever develop in these,
but the great arteries run to them, and indeed to all parts of the
embryo, on almost precisely the same general plan as in the adult
fish. Only later is the definite avian circulation gradually
acquired.

This law is even more strikingly illustrated in the embryonic
development of the vertebral column and skull, if we had time to
trace their development. And the development of the excretory system
points to an ancestor far more primitive than even the fish. Our
embryonic development is one of the very strongest evidences of our
lowly origin.

Thus we have three sources of information for the study of animal
genealogy. First, the comparative anatomy of all the different
groups of animals; second, their comparative embryology; and third,