Chapter 6 

 Variation and heredity 



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

 ism vary in morphological and physiological characteristics. Pro- 

 tozoa are no exception, 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. Wesen- 

 berg-Lund (1908) noticed a definite seasonal morphological variation 

 in Cerctium hirundinella in Danish lakes, while Schroder (1914) 

 found at least nine varieties of this organism (Fig. 94) occurring in 

 various bodies of water in Europe, and List (1913) reported 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 inhabiting the same deep lakes; 

 namely, individuals from the deep water may reach 200m in length, 

 while those from the surface layer measure only about 100^ 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, Orbulina universa inhabiting surface water has a com- 

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

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

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

 distinct dimensional difference in different springs. It measured 

 10— 40/x in diameter in sulphurous water and 45-80^ 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 usually dis- 

 appear 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 al- 

 ready been mentioned (p. 109). Chlamydomonas debaryana are repre- 

 sented by many races differing in form, size, and structure, in various 

 localities as well as under different laboratory conditions. Moewus 



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