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 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, their palaeontological history.  Each source has its difficulties or defects.  But taken all together they give us a genealogical tree which is in the main points correct, though here and there very defective and doubtful in detail.  The points in which we are left most in doubt in regard to each ancestor are its modes of life and locomotion, and body form.  But these may temporarily vary considerably without affecting to any great extent the general plan of structure and the line of development of the most important deep-seated organs.