502 



THE CHANGING WORLD 



amphibiocns 



fis'hes 



Earlier Agassiz (1807-1873) pointed out that the embryo of higher 

 forms does not so much resemble the adults of lower living forms as 

 mammals it does adults of lower fossil forms. 



I birds Von Baer (1792-1876), "Father of 



Embryology," maintained that the 

 resiles j^Qj-g nearly the adults of two groups 

 resemble each other, the longer their 

 embryonic development follows an 

 identical path. To the evolutionist 

 resemblance of this kind means rela- 

 tionship. This concept is diagram- 

 matically shown in the figure at the left 

 in which it is evident that mammals, 

 for instance, are more nearly related 

 to reptiles than to fishes, because they 

 tread the embryological road together 

 for a longer period. 



Morgan (1866- ) proposed a 

 "Repetition Theory," in which the 

 embryonic stages run along parallel lines, rather than in one com- 

 posite line, to diverge eventually into adult stages. This idea pre- 

 cludes the possibility that the embryonic stages in any one group 

 could be represented by adult ancestral stages of another group. 

 The adult mammal, for instance, does not look back upon the adult 



I 



I ^ 



. bircCs I ^ 



reptilss \ '" 



. amphibians \ ^ 

 d 



-["ishes 



s\ 



i 



fishes amphibians nsptiks birsls mammals 



Diagram illustrating ontogeny and phylogeny. 



fish as one of the embryonic stages through which it has passed, but 

 apparent resemblances are to be accounted for on the ground that 

 the development of the mammal runs parallel with that of the fish, 

 and consequently resemblances are to be expected. 



