CHAPTER TWENTY-FIVE 



PROTOZOA AND HEREDITY 



Races of I- THE species of fresh-water Protozoa, widely dis- 



Protozoa tributed over the earth, are remarkably constant in 

 what we call their specific characters. When minutely 

 studied, however, they are found to vary within rather 

 wide. limits, and it appears that there exist numerous 

 minor races, too much alike to be recognized as distinct 

 species. Professor H. S. Jennings of Johns Hopkins 

 University has shown that the intensive study of these 

 minute animals will yield results of the highest interest 

 in connection with the problems of heredity. The 

 slipper animalcule, Paramecium (plural, Paramecia), is 

 a ciliated form extremely common in water containing 

 decomposing vegetable matter. It is transparent, so 

 that all its characters can be readily observed, while its 

 rapid rate of reproduction makes it possible to follow 

 it through numerous generations. As it does not neces- 

 sarily conjugate, but is capable of reproducing for very 

 long periods, if not indefinitely, by simple division, it is 

 possible to eliminate the confusion due to biparental 

 inheritance. 



Pure lines "Pure lines" can be obtained, all directly descended 

 from a single ancestor. Such pure lines have members 

 with identical hereditary composition, although in- 

 dividuals may show conspicuous differences due to en- 

 vironmental conditions. Even in a watch glass it is 

 impossible to make the conditions absolutely uniform. 

 The lower layers of the water are likely to contain 

 accumulations of bacteria, which are injurious to the 

 protozoans. Individuals entering the less favorable 

 surroundings will have their vitality somewhat im- 

 paired, and thus they show less energy in swimming to 



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