Inheritance of Variations 



71 



Figure 23. Difflugia corona, to show the characters studied in the 

 work on inheritance. A. Side view. B. Oral view, showing the mouth 

 and the teeth surrounding it. After Jennings, 1916. 



the length of the spines ; the number of the teeth about the 

 mouth (Figure 23, B) ; and the size of the shell, as meas- 

 ured by its diameter. All these characters are formed when 

 the individual is produced (Figures 11 and 12), and are not 

 subject to change by growth, nor are they altered by changes 

 in the environment during the life of the individual. No 

 more favorable combination of characters for the study of 

 variation and heredity could possibly be found. 



Difflugia is extremely fastidious as to just how it shall 

 live and what sort of food it shall be furnished, so that it 

 is not easy to keep pedigreed stock for generation after 

 generation, as is necessary for all work in heredity. When 

 these difficulties are overcome, we follow for long periods the 

 history of a given race. As we have seen, when we compare 

 different races, the peculiarities of the parents reappear in 

 the offspring in a high degree (Figures 19 to 21). But di- 

 versities do arise even within a single race. Parents with 

 many spines have as a rule progeny with many spines, but 



