Chapter 6 

 Variation and heredity 



IT IS generally recognized that individuals of all species of organ- 

 ism show a greater or less variation in morphological and physio- 

 logical characteristics. Protozoa are no exceptions, and manifest a 

 wide variation in size, form, structure, and physiological characters 

 among the members of a single species. The different groups in a 

 species are spoken of as the races, varieties, strains, etc. It is well 

 known that dinoflagellates show a great morphological variation in 

 different localities. Schroder (1914) found at least nine varieties of 

 Ceraiium hirundinella (Fig. 85) occurring in various waters of 

 Europe, and List found that the organisms living in shallow ponds 

 possess a marked morphological difference from those living in deep 

 ponds. Cyphoderia ampulla is said to vary in size among those in- 

 habiting the same deep lakes; namely, individuals from the deep 

 water may reach 200m in length, while those from the surface layer 

 measure only about 100m long. 



In many species of Foraminifera, the shell varies in thickness ac- 

 cording to the part of ocean in which the organisms live. Thus the 

 strains which live floating in surface water have a much thinner shell 

 than those that dwell on the bottom. For example, according to 

 Rhumbler, Orhulina universa inhabiting surface water has a com- 

 paratively thin shell, 1.28-18m thick, while individuals living on the 

 bottom have a thick shell, up to 24m in thickness. According to 

 Uyemura, a species of Amoeba living in thermal waters, showed a 

 distinct dimensional difference in different springs. It measured 

 10-40m in diameter in sulphurous water and 45-80m in ferrous water; 

 in both types of water the amoebae were larger at 36-40°C. than 

 at 51°C. ' 



Such differences or varieties appear to be due to the influence of 

 diverse environmental conditions, and will continue to exist under 

 these conditions; but when the organisms of different varieties are 

 subjected to a similar environment, the strain differences disappear 

 sooner or later. That the differences in kind and amount of foods 

 bring about extremely diverse individuals in Tetrahymena vorax and 

 Chilomonas Paramecium in bacteria-free cultures has already been 

 mentioned(p. 94). Chlamydomonas dcbaryana are represented by 

 many races differing in form, size, and structure, in various localities 

 as well as under different laboratory conditions. Moewus (1934) dis- 



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