584 H. S. 'Jennings 



assumed difference in heredity between unicellular and 

 multicellular animals — the "inheritance of acquired 

 characters" 



It is often said, and it seems to be generally assumed, that uni- 

 cellular animals differ fundamentally from multicellular ones in 

 heredity.* In the Protozoa there is no separation into cells which 

 normally die after a certain period ("somatic" or "body" cells), 

 and cells which continue to live and multiply ("germ" cells). The 

 parent produces progeny by simply dividing, so that parents and 

 progeny are identical. 



This seems to simplify extremely the problem of heredity, or 

 indeed to remove everything problematical from the subject. 

 Parents and progeny must be alikey it is said, because they are 

 the same. In particular it is commonly held that this removes 

 from the Protozoa all difficulty as to the "inheritance of acquired 

 characters" — characters added during the lifetime of the indi- 

 vidual and due to environmental action, experience, use, accident, 

 or the like. Such characters are in multicellular organisms often 

 called somatic, as distinguished from germinal, and such somatic 

 characters are commonly held not to be inherited. Where there 

 is no such distinction between soma and germ, it would seem clear 

 that there can be no distinction between somatic and germinal 

 characteristics. 



To this difference in heredity between Protozoa and Metazoa 

 much importance has been attached. If the difference really 

 exists, the Protozoa are much more plastic in evolution than are 

 the Metazoa; through the inheritance of the effects of experience, 

 use and environment, the Protozoa must permit of the ready and 

 rapid production of varied and adapted types. This point has 

 been emphasized by many writers. For example, in attempting 

 to account for the great diversities of organization and action 

 found among animals Whitman ('99) writes as follows: 



"In primitive organisms multiplying by simple fission, struc- 

 tural modifications acquired during the lifetime of the individual 



*I use the word ''heredity" merely as a brief and convenient term for "the resemblance between 

 parents and progeny,'' without implying any underlying entity, and without prejudice as to the 

 grounds of this resemblance. 



