i22 POPULAR SCIENCE MONTHLY. 



disappear long before birth. These indications, 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 intermediate stages are also 

 inheritances, and, therefore, represent phases in ancestral history? 

 If they be, indeed, clues to ancestral conditions, may we not, by patch- 

 ing together our observations, be able to interpret the record, just as 

 the history of ancient peoples has been made out from fragments in 

 the shape of coins, vases, implements, hieroglyphic inscriptions, etc.? 



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 de- 

 gree phases of their ancestral history. This is one of the widest gen- 

 eralizations of embryology. It was suggested in the writings of Von 

 Baer and Louis Agassi z, but received its first clear and complete ex- 

 pression in 1863, in the work 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 adaptation of a particular organism to its condi- 

 tions of development, 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. 



Widely spread through the recent literature is to be noted a re- 

 action against the too wide and unreserved application of this doc- 

 trine. This is to be naturally expected, since it is the common 

 tendency in all fields of scholarship, to demand a more critical esti- 

 mate in research, and to undergo a reaction from the earlier crude an I 

 sweeping conclusions. 



Improvement in Methods. — Another feature of the work in Bal- 

 four's period was increasing attention to methods of preparing ma- 

 terial for study. The great problem is to bring tissues under observa- 

 tion, with the normal relations as little disturbed as possible,* so that 

 the prepared material will represent the conditions existing in life 

 and no others. " Many of the most important elements of cell- 

 structure are invisible in life, and can only be brought to view by 

 means of suitable fixation, staining and clearing." One great danger 

 is that pseudo-structures will be artificially formed by the action of 

 reagents. On this account great attention has been given to every 



