572 BIOLOGY OF THE PROTOZOA 



structure does not change after it is once formed. In such a study 

 Jennings says the method of evohition by slow and gradual change 

 rather than by sudden jumps or mutations becomes visible. "We 

 begin to exercise selection within the single family. On the one 

 hand we select all the long-spined individuals and place them 

 together; on the other hand we select all the short-spined ones and 

 place them together. In the long-spined group we continue to 

 save for generation after generation only the individuals that are 

 long-spined; in the short-spined group only the offspring with short 

 spines. In the same way we select other sets for numerous spines 

 and for few spines; for large shells and for small shells; for many 

 teeth and for fewer teeth. 



"And now as we keep this up for generation after generation we 

 find that the correspondence between parent and progeny becomes 

 more and more marked. We find that our single family is breaking 

 up into many different groups which differ from one another heredi- 

 tarily. We get finally what appears to be two diverse races- 

 one with long spines, the other with short spines— the difference 

 continuing for generation after generation. A third set has con- 

 stantly large shells, while others consistently produce small shells. 

 We also get stocks hereditarily different for numbers of spines; and 

 for numbers of teeth. Our single stock derived by fission from a 

 single parent, has gradually diversified itself into many stocks that 

 are hereditarily different. If this is what we mean by evolution, we 

 have seen evolution occur" (Jennings 1921, pp. 75-78). 



In a similar manner Root (1918) and Hegner (1918) studied 

 uniparental inheritance in Centropyxis aculeata and in Arcella 

 dentata and obtained results of the same nature. External agents 

 (lack of food, salts, temperature, etc.) may bring about similar 

 variations in size of shell, numbers of spines, etc., which persist as 

 long as the conditions are maintained (Hegner, 1919). From this 

 it appears that external conditions may inhibit the expression of 

 germinal factors, but not permanently. 



The interpretation as given by Jennings of these clear-cut results 

 appears to be fundamentally sound and its significance is not les- 

 sened by the chromidia problems which are associated with all of 

 these testate rhizopoda. If as generally believed, the chromidia 

 give rise to germ nuclei there is some chance of this hereditarily 

 important chromatin being unequally distributed at cell division, 

 for the mass of chromidia is not halved with the same precision as 

 is the chromatin of the nucleus or nuclei. Whether or not chromidia 

 are responsible the interesting fact remains that demonstrable 

 variations in organization occur with continued reproduction. It 

 remains to be determined, however, whether the variations will 

 still breed true after endogamous fertilization and reorganization, 

 or will revert to the form of the original wild individual; then only 



