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a- certain pressure of the oxygen, else, these characters are seen 

 to disappear, whilst in fact, or apparently, new ones originate. Hence, 

 in some cases it may be proved, in others the probability is shown, 

 that each species must occur in three varieties, joined by intermediate 

 forms, i. e. the normal form, n "high pressure variant", and a "low 

 pressure variant". 



As in wholly different groups of bacteria corresponding facts may 

 be observed, there is cause to assign a fundamental signification 

 .to them. 



A decisive factor which may cause the production of variants is 

 furthermore the temperature, for experience proves thai a prolonged 

 cultivation above the optimum temperature of growth, gives rise 

 to the appearance of forms distinctly different from the original stock, 



In other cases the cause of the variability is unknown: not seldom 

 for example, we find at the very first culture of a species taken from 

 nature, strongly varying colonies, which prove to belong to the same 

 species only because many colonies by sector-variation display the 

 genetic alliance of the variants to the wild stock. 



But then, too, there is reason to admit that the new vital condi- 

 tions, to which the microbes are subjected just by the change of 

 oxygen pressure and temperature, are the chief factors of the variation 

 process which is, as it were, seen in action. This observation is of 

 so general a nature and is so closely related to the essence of life, 

 that it must be considered as probable, that also in higher plants 

 and animals, local changes in the access or exclusion of oxygen, in 

 connexion with temperature, play an important part in the morpho- 

 genesis. 



As the examination of other species of microbes shows thai the 

 absence of certain nutrient substances in the culture medium, at free 

 aeration and during growth, may cause hereditary variation, for example 

 in Schizosaccharomyces octosporus, which in old cultures changes into 

 the spore-free variant, totally differing from the chief form, there is 

 reason also to believe, thai also the said factor must be considered 

 to explain the great variability of the lactic acid ferments; but the 

 observations there about are not yet tit for definite conclusions. 



3. Elective culture of the microbes of the slimy 

 luetic acid fermentation. 



There is reason to assume that the slime producing lactic acid ferments 



are the normal forms and the non-sli tonners, species or variants 



derived from them. Hence, the former deserve to be considered in 

 the first place. 



