INHERITANCE OF ACQUIRED CHARACTERS 179 



which were responsible for the tendency of the organism 

 to develop in the given direction were already present 

 in the germ-plasm. As Walter aptly remarked, " Not 

 only the development of the race which we call evolution, 

 but also the determination of the individual in heredity, 

 is a chain of onward-moving sequences like the succession 

 of events in history. It is hard to see how recent events 

 can influence preceding events. It is hard to see how 

 the water that has gone over the dam can return and 

 affect the flow of the river upstream in any direct way. 

 It is likewise hard to see how differentiated somatoplasm, 

 which represents the end stage of a successive series of 

 modifications, can make any definite impress upon the 

 original germplasmal sources from which it arose." 1 



Even Darwin found difficulty in believing in the inheri- 

 tance of acquired characters. His theory of pangenesis 

 which assumed that each somatic cell added to the cir- 

 culation a minute granule which later found its way to 

 the germ-plasm is not substantiated by later investiga- 

 tions. 



The evidence presented to prove that somatic modi- 

 fications are actually transmitted from parent to offspring 

 is not conclusive. It is certain that acquired characters 

 cannot be transmitted unless the germ-plasm has been 

 definitely changed by reason of somatic influence. The 

 evidence of such influence upon the germ-cell is entirely 

 negative. Definite experiments carefully planned for 

 testing the possible effect of such influence have been 

 inconclusive. 



166. The inheritance of disease. The earlier writ- 

 ings on animal breeding contain numerous references to 

 the possibility of inheritance of disease. Many examples 



1 Walter, "Genetics," 1913, p. 85. 



