JENNINGS: CHANGES IN HEREDITARY CHARACTERS 285 



there is nothing which complicates genetic problems so enor- 

 mously as does the continual mixing of diverse stocks in biparental 

 inheritance. In uniparental reproduction we have but one 

 genotype to deal with; we can be certain that no hereditary 

 characters are introduced from outside that genotype. 



To hope for results on the problem in which we are interested, 

 we must resolve to carry on a sort of second degree research, as 

 it were. That is, we must accept as a foundation the facts 

 before discovered, as to the make-up of the species out of a great 

 number of diverse stocks ; as to the usual effects of selection being 

 nothing save the isolation of such preexisting stocks. What 

 we must do is to take a single such stock — choosing an organism 

 that is most favorable for such work — then proceed to a most 

 extensive and intensive study of heredity, of variation, and of 

 the effects of selection for long periods within such a stock. 



Such an organism, most favorable from all points of view, I 

 found in the rhizopod Difflugia corona. It has numerous dis- 

 tinctive characters, all congenital; all inherited in a high degree; 

 yet varying from parent to offspring also; none of these char- 

 acters changed by growth or environmental action during the 

 life of the individual. 



Long continued work showed that a single strain of this animal, 

 all derived by fission from a single parent, does differentiate 

 gradually, with the passage of generations, into many hereditar- 

 ily diverse strains. The important facts about the hereditary 

 variations and their appearance are the following: 



1. Hereditary variations arose in some few cases by rather 

 large steps or "saltations." 



2. But the immense majority of the hereditary variations 

 were minute gradations. Variation is as continuous as can be 

 detected. 



3. Hereditary variation occurred in many different ways, in 

 many diverse characters. There was no single line of variation 

 followed exclusively, nor in the overwhelming majority of cases. 



4. It gave rise to many diverse combinations of characters: 

 large animals with long spines; small animals with long spines; 

 large animals with short spines; small animals with short spines; 



