138 



GENETICS 



In bacteria it is possible to isolate out variants 

 from a single strain but in none of the cases is the 

 origin of the supposed "clone" unquestionably from a 

 smgle bacterium as it would need to be in order to 

 form a pure line, so that what has occurred in all prob- 

 ability is the simple assortment of a pure line from a 

 population. 



Among the protozoa, which reproduce asexually by 

 fission, much painstaking experiment and observation 



95 



FIG. 28. Eight pure races of Paramecium. The actual mean 

 length of each race is given in micra below the corresponding 

 outline. Magnified about 230 diameters. After Jennings. 



has been made, notably by Jennings and various investi- 

 gators whom he has inspired. 



For example, Jennings found that Paramecia differ 

 from each other in size, structure, physical character, 

 and rate of multiplication as well as in the environ- 

 mental conditions required for their existence and, 

 furthermore, that these differences, in an hereditary 

 sense, are "as rigid as iron." 



With respect to the character of mean length he 



