26 THE WHENCE AND THE WHITHER OF MAN 



and must remain short. Hence in higher forms the 

 ancestral stages will often be slurred over and very in- 

 completely represented. And the embryo may, and 

 often does, shorten the path by " shortcuts " impossi- 

 ble to its original ancestor. Still it will in general 

 hold true, and may be recognized as a law of vast im- 

 portance, 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 



