54> Life and Death, Heredity and Evolution 



and allow them to reproduce, we find that the offspring re- 

 peat in large measure the peculiarities of their parents 

 (Figure 20). That is, the particular characteristics of the 

 parents are inherited, quite independently of diversity in 

 conditions. If we allow each of the diverse parents to re- 

 produce for generation after generation, we find that in each 

 case the peculiarities of the original stock are retained 

 (Figure 21). That is, each single species, such as this one 

 of Difflugia corona, consists of a large number of hered- 

 itarily diverse strains or families ; of strains remaining di- 

 verse for generation after generation. 



This is one of the facts of capital importance in the 

 biology of these organisms ; something that has to be kept 

 continuously in mind in all attempts to work with them or 

 to understand them. It forms the key and explanation for 

 many remarkable phenomena in their lives. We shall there- 

 fore look at the concrete facts for a number of typical cases, 

 and examine their results in relation to heredity and varia- 

 tion. 



In Difflugia corona the number of hereditarily diverse 

 strains that have been found is indefinitely great. They 

 differ in size and form, in the number of spines, in the length 

 of the spines, in the number of teeth surrounding the mouth. 

 Different strains have hereditarily different combinations of 

 these characters ; some have large shells with few spines ; 

 others have large shells with many spines, and so on for 

 other combinations. 



Besides these marked structural peculiarities, the strains 

 of Difflugia differ in many other ways not apparent to the 

 eye. Some of the strains multiply rapidly, others slowly. 

 Some are very hardy and easily cultivated in the laboratory ; 

 others are delicate, dying out under artificial conditions. 

 Some are very active, others quiet. Some are adapted to 



