EMBRYOLOGY AND EVOLUTION 



embryology," recognized that this view was not wholly cor- 

 rect, and he modified it as follows: "An embryo never 

 resembles an adult animal and is only to be compared with 

 the embryos of other animals. The more different two 

 animal forms are in their end stages the farther back in their 

 development must one go in order to find agreement between 

 them." This has often been called "von Baer's Law." 

 Louis Agassiz, in his famous Essay on Classification (1858), 

 pointed out the fact that there is a parallelism between 

 embryology, palaeontology, and classification in that the 

 stages that an animal passes through in its development from 

 the tgg resemble certain animal forms that have appeared in 

 the past history of the earth and also certain lower forms 

 now living. 



The full significance of this parallelism was not appre- 

 ciated until the revival of the doctrine of evolution under 

 Darwin. In the fourteenth chapter of the Origin of Species 

 Darwin discusses this parallelism and the significance of 

 the homologies of embryos, and he closes his discussion of 

 embryology with these carefully guarded words: "Embry- 

 ology rises greatly in interest when we look at the embryo 

 as a picture, more or less obscured, of the progenitor, either 

 in its adult or larval state, of all the members of the same 

 great class." 



It was Ernst Haeckel, in his Generelle Morpbologie 

 (1866) and in many later books, who announced that 

 "Ontogeny is a short recapitulation of Phylogeny" — that is, 

 the successive embryonic stages in the development of an 

 animal correspond to the successive adult stages of the 

 phylum to which it belongs. This is Haeckel's Fundamental 

 Law of Biogeny ("Biogenetisches Grundgesetzt"), which is 

 more frequently called the theory of embryonic recapitula- 

 tion. 



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