XV 



DEVELOPMENT 



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ditions of growth and development. The suggestion of 

 Kleinenberg referred to in a preceding chapter helps us, 

 for if we ask why an animal develops a notochord only 

 to have it rapidly replaced by 

 a backbone, part of the answer 

 surelv is that the notochord 



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which in the historical evolution 

 supplied the foundation neces- 

 sary for the evolution of a back- 

 bone, is still necessary in the 

 individual history for its de- 

 velopment. 



It may be said that recapitu- 

 lation is more clearly seen in 

 the stages in the development 

 of organs (organogenesis) than 

 in the development of the or- 

 ganism as a whole. It must 

 be remembered, moreover, that 

 an organism is specific, itself 

 and no other from beginning 

 to end. Thus although the 

 frog illustrates recapitulation 

 in its life-history, e.g. in the 

 development of the heart and 

 the circulation, and clearly re- 

 veals its piscine ancestry, it is 

 specifically an Amphibian from 

 first to last and through and 

 through. The student should 

 refer to Herbert Spencer's Prin- 

 ciples of Biology, where the 

 recapitulation idea is discussed 

 in connection with the general 

 idea of evolution as a progress 

 from the homogeneous to the 

 heterogeneous. About the same time (1866) Haeckel 

 gave vivid illustrations of the recapitulation idea, stating 

 it in his ' fundamental biogenetic law " : " Ontogeny, 

 or the development of the individual, is a shortened 



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