204 METAMORPHOSIS AND 



velopment simply because it has never been lost.' 

 He concludes therefore that the gill slits of the 

 embryo Bird represent the gill slits of the embryo 

 Fish, and not the adult gill slits of the Fish, which 

 have been in some mysterious way pushed back into 

 the embryo of the Bird. 



Morgan evidently does not realise that the Birds 

 and Reptiles must have been derived from Amphibia, 

 and that the embryo Reptile or Bird with gill slits and 

 gill arches is merely a tadpole enclosed in an egg shell. 

 The Frog in its adult state differs much from a Fish, 

 while the larva in its gill arches and gill slits resembles 

 a Fish. Morgan contends that the new characters 

 do not add themselves to the end of the line of 

 already existing characters. But in the case of the 

 Frog this is exactly what they have done. The exist- 

 ing characters were in this case the gill arches and 

 slits. Those who believe in recapitulation do not 

 suppose that the animal had to live a second life 

 added on to the life of its ancestors and that the 

 new characters appeared in the second life. They 

 believe that in the ancestor a certain character or 

 general structure of body when developed persisted 

 without change throughout life like the gill arches 

 and slits in a Fish. At some stage of life before 

 maturity this character underwent a change, and in 

 the descendants the development of the original 

 character and the change were repeated by heredity. 

 There is no ' mysterious pushing back of adult 

 characters into the embryo,' although it is possible or 

 even probable that in some cases the change gradu- 

 ally became earlier in the life history : it is the new 

 character which is pushed back, not the adult 

 character of the ancestor. 



