4 THE SCOPE OF THE ENQUIRY [CH. i 



and to defend their position there by the manufacture of 

 haemolysins and toxins. 



This process of evolution is, no doubt, going on continually 

 in bacteria as in higher forms of life. It is rendered possible 

 in both cases by the occurrence of natural variation. This 

 variation in bacteria is of two kinds, namely, spontaneous or 

 intrinsic variation between the individuals of a pure culture 

 that is to say, bacteria derived from a single organism, and 

 impressed variation, the effect of special environmental con- 

 ditions upon a succession of bacterial generations, due either 

 to the direct reaction of the bacterial protoplasm to the 

 environment, or to selection acting upon slight spontaneous 

 variations and producing a cumulative effect. 



It is reasonable to expect that amongst the bacteria natural 

 variation would occur with greater frequency than amongst 

 higher forms of life for, being unicellular organisms, changes 

 in their environment can operate directly upon the germ plasm. 

 Moreover the common method by which bacteria multiply, 

 namely the division of the parent cell into two daughter cells, 

 ensures the ready transmission of any acquired character from 

 parent to offspring. The variation in character may be said to 

 be retained by the daughter cells rather than transmitted to 

 them. Such retention of parental characters by the daughter 

 cells is not, however, invariable. For example, McDonald 

 (1908) has published photographs of a young culture of the 

 meningococcus showing diplococci in which one member is 

 stained while the other is not. Thirdly, such variations would 

 be more readily noted in their case since as many as 30 or 

 40 successive generations may be observed in the course of 

 24 hours. In the case of some bacteria division may occur as 

 frequently as once every 17 minutes (Barber, 1908). Yet a 

 fourth factor might be mentioned, namely the ease with which 

 the environment of any strain of organisms can be modified in 

 any direction and to any extent. 



As a matter of fact examples of such variation, as we shall 

 show, are innumerable, no matter what particular property or 

 character of bacteria we investigate. Differences occur in size 

 and shape, in staining properties, in power of growth on various 



