230 BIOLOGY AND ITS MAKERS 



pearancc, but disappear long before birth. These indica- 

 tions, and similar ones, must have some meaning. 



Now whatever qualities an animal exhibits after birth 

 are attributed to heredity. May it not be that all the inter- 

 mediate stages are also inheritances, and, therefore, represent 

 phases in ancestral history? If they be, indeed, clues to 

 ancestral conditions, may we not, by patching together our 

 observations, be able to interpret the record, just as the his- 

 tory of ancient peoples has been made out from fragments 

 in the shape of coins, vases, implements, hieroglyphics, in- 

 scriptions, etc. ? 



The Recapitulation Theory.- -The results of reflection in 

 this direction led to the foundation of the recapitulation 

 theory, according to which animals are supposed, in their 

 individual development, to recapitulate to a considerable 

 degree phases of their ancestral history. This is one of the 

 widest generalizations of embryology. It was suggested in 

 the writings of Von Baer and Louis Agassiz, but received its 

 first clear and complete expression in 1863, in the writings of 

 Fritz Muller. 



Although the course of events in development is a record, 

 it is, at best, only a fragmentary and imperfect one. Many 

 stages have been dropped out, others are unduly prolonged 

 or abbreviated, or appear out of chronological order, and, 

 besides this, some of the structures have arisen from adapta- 

 tion of a particular organism to its conditions of develop- 

 ment, and are, therefore, not ancestral at all, but, as it were, 

 recent additions to the text. The interpretation becomes a 

 difficult task, which requires much balance of judgment and 

 profound analysis. 



The recapitulation theory was a dominant note in all 

 Balfour's speculations, and in that of his contemporary and 

 fellow-student Marshall. It has received its most sweeping 

 application in the works of Ernst Haeckel. 



