CHAPTER VIII 

 VARIATION OF SPECIES 



THIS is a rather intricate question to consider. The characters 

 which we have just studied may be utilized for the determina- 

 tion of a species when they are fixed and do not vary within the 

 species. This involves the whole question of constancy of characters. 

 Are such characteristics absolutely constant or only relatively constant? 

 To what extent may they vary? Do well-determined varieties of 

 yeasts exist or may they change with the environment? Finally, if these 

 variations occur, are they permanent or simply transitory? There is 

 the important question, for if the characteristics upon which we would de- 

 termine a yeast vary, all hope of differentiating species becomes illusory. 



Again it is Hansen l who has contributed the most to elucidate 

 this problem by showing that yeasts may undergo more or less im- 

 portant variations; some permanent, others transitory. In some cases 

 changes have taken place which have been of such nature that the 

 yeast has not returned to its former state, even after attempts cover- 

 ing a number of years. Let us consider some of these variations which 

 permit the determination of species and separate them from each 

 other. Some will be found to be more constant; others variable. We 

 shall distinguish variations in shape (morphological) from variations 

 in function. In this category, we shall separate the variations which 

 are temporary from those which are permanent. Certain it is that 

 such a division is arbitrary because modifications in morphology are 

 always accompanied by modifications of physiological activities. It 

 is well to adopt it, however, for the convenience of exposition. 



Morphological Variations : Polymorphism : Yeasts are quite poly- 

 morphic and may show different shapes in the same culture. This 

 may depend upon the conditions which surround the yeasts. 



If one inoculated, for example, a single cell into a nutrient me- 

 dium, it would be found that many different cells would develop 

 from this single cell; from this, it is seen that the yeasts have no 

 definitely constant shape. With Saccharomyces cerevisiae it has been 



1 Hansen, E. C. Experimental studies on the variations of yeast cells. Read 

 before the Botanical section of the British Association. Ipswich, Sept. 13, 1895. 

 Annals of Botany, 9, 1905; Ueber die Variation bei den Bierhefepilzen und bei 

 anderen Saccharomyceten. Zeit. Brauw. 21, 1898. Cent. Bakt. Abt. II, 4, 1898; 

 Recherche sur la phys. et la morphologic des ferments alcool. C. R. des trav. du 

 lab. de Carlsberg, 5, 1900. 



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