290 BACTERIAL VARIATION 



the underlying mechanisms involved, it seems wiser to adhere to the older name, 

 which serves our purpose well because of its very vagueness. 

 Correlated Variations. 



An observed variation in any given bacterial character clearly gains in signific- 

 ance if it is found to be uniformly or frequently associated with a change in some 

 other character, or characters. It is a fairly safe assumption that correlated 

 variations of this kind indicate some relatively major change in genetic make-up. 

 Whether the correlated character changes are different expressions of a single 

 character factor, or are due to changes in two or more genetic factors that are 

 themselves associated as a result of the reproductive mechanisms of the cell, 

 we cannot tell ; but in a few instances, as in the smooth — >■ rough variations to 

 which we have already referred in Chapter 8, and which we shall shortly describe 

 in more detail, we can relate many of the associated character changes to a loss of 

 the ability to synthesize and store a particular chemical component of the bacterial 

 cell. 

 Impressed Variations. 



By an impressed variation is meant a variation that occurs in response to a 

 particular environmental stimulus ; so that, by applying the stimulus, we can 

 induce the variation at will. It should be noted that, if this term is to be applied 

 in its strict sense, it is the genetic variation that must be impressed, not merely the 

 character change by which this variation is recognized. We know, for instance, 

 certain genetic variations in insects that lead to the appearance of well-defined 

 character changes under particular environmental conditions. In the absence of 

 these conditions the insects appear to conform to the normal type, though the 

 genetic differences, which are themselves quite independent of these environmental 

 factors, persist all the time. The criteria that justify the conclusion that a parti- 

 cular variation has been induced by a particular environmental stimulus are clearly 

 that the application of the stimulus should regularly be followed by the appearance 

 of the variation in question, and that the variant form should persist, over many 

 generations at least, after the stimulus has been withdrawn. The appearance of 

 a variant form in response to a given stimulus, followed by immediate reversion to 

 the normal form when the stimulus is no longer applied, should be regarded as a 

 temporary adaptation to a changed environment, rather than as a variation in 

 the sense in which that term is used here. It is often extremely difficult, in the light 

 of our present knowledge, to determine in which category a given change in bacterial 

 form or function should be placed. 



It will be convenient to discuss the variability of each of the more important 

 bacterial characters in turn, indicating, where possible, whether the variation in 

 question is correlated with others, and whether it occurs naturally or in response 

 to any known change in environment. 



Variations in Morphology. 



There are innumerable accounts in the literature of changes in shape, size or 

 structure of bacterial cells. Some of these, such as the involution forms that appear 

 in cultures of the plague bacillus when that organism is grown on agar with a high 

 salt-content, are clearly a direct response to an environmental stimulus, and are 

 not inherited. In other instances it is very difiicult to tell whether or not true 

 variation has occurred. From among the many examples available, we may select 

 the following as illustrating the kinds of variation that have significance from our 

 present point of view. 



