112 MARINE BIOLOGICAL LABORATORY. 



which has been acquired through natural selection dur- 

 ing the evolution of the Metazoa from the Protozoa. 

 Whether this conclusion is just is to a certain extent 

 a side-issue to the theory of the continuity of germ- 

 plasm, and further on the matter will be more fully 

 discussed. 



Among the Protozoa it is very evident that the 

 young must resemble the forms from which they 

 spring, for they are a part of the same stuff. Further- 

 more, if during life an individual becomes changed by 

 its environment, this change must become an hereditary 

 trait. 



Now we may assume that the Protozoa may be 

 affected by the physical conditions under which they 

 live just as much as are the Metazoa. It is a well- 

 known fact that the bodies of the higher forms may 

 be changed by food and exercise. The muscles of a 

 blacksmith or a sailor are stronger than those of a dude. 



A Paramaecium which lives in a strong-flowing stream 

 would be apt to acquire stronger cilia than one living 

 in a stagnant pool. The young which spring from a 

 Paramaecium with strong cilia must inherit this charac- 

 ter. Indeed, the cilia of the young are the identical 

 cilia which by constant use have been lengthened and 

 strengthened. In every case of reproduction by fission, 

 the whole body of the parent continues its existence 

 in the younger generation. If the parent is strong and 

 vigorous, the young are so likewise ; if the parent is 

 decrepit and feeble, the young must likewise be de- 

 crepit and feeble. Hence all modifications which any 

 unicellular organism may acquire must be transmitted 

 to its ySung. 



