586 H. S. 'Jennings 



the fate of new structural characters ("acquired 

 characters") 



As we have just seen, it is commonly held that "acquired char- 

 acters" are inherited in the Protozoa, though not in the Metazoa. 

 Do experiment and observation show that this is true ? Does 

 the separation of germ and body cells make a fundamental dif- 

 ference in heredity ? 



I Localized and Unlocalized Characteristics 



In dealing with new or "acquired" characters, it is well to dis- 

 tinguish two classes. On the one hand are those characters 

 (mainly structural), that are localized in a definite part of the 

 body, as cilia, setae, a mouth, etc. On the other hand there are 

 characters that affect the organism as a whole; such are acclim- 

 atization or other general modifications due to heat, cold, chemical 

 agents, etc.; size, method of growth, and the like. The inheritance 

 of the latter class of characteristics, however acquired, presents 

 much less apparent difficulty than does the inheritance of the 

 former. The importance of this distinction between localized and 

 unlocalized characteristics, in investigations of heredity, has 

 often been emphasized. Weismann has repeatedly demanded 

 as proof of the inheritance of somatogenic characters in Metazoa 

 a demonstration of "the transmission of changes of single definite 

 parts of the parents to the corresponding parts of the progeny;" 

 of the inheritance of "definite parts or localized functions." It 

 is clear that a somewhat different problem is involved in the two 

 classes of cases. We shall take up first localized characters in 

 the Protozoa. 



2 Typical Examples of Inheritance and its Problems in Protozoa 



To appreciate the problem of the inheritance of localized char- 

 acters, we will look at one or two simple cases in the Protozoa; 

 these will serve to bring the whole problem of inheritance in these 

 animals to a point. 



Paramecium (Fig. i) has a blunt anterior end and a pointed 

 posterior end. How does it happen that after fission similar 



