108 ON VARIABILITY AND ADAPTATION 



morphism " and round Nageli's theory as to variability, 1 for at 

 the time that these were most discussed the subject was not 

 ripe for treatment. The methods of investigation were imperfect, 

 pure growths could not with certainty be obtained, and con- 

 sequently the arguments advanced pro and contra rested upon 

 insecure bases. Still Lankester's observations 2 on the change 

 of form of the Clathrocystis, or Beggiatoa, roseo-persicina, 

 and later Kurth's " Observations on the Bacillus Zopfii," 3 have 

 stood the test of time, and may be said to have laid a solid 

 foundation upon which rests our present knowledge of variation 

 among microbes. 



Transient Variability 



We possess now such a mass of separate observations upon 

 transient variability of bacteria that it is difficult to know how 

 much to say on this subject here, how much to leave unsaid. 

 Every one who of late years has studied bacteriology must have 

 come across numerous cases of this nature. Every one admits 

 that according to the age of a growth, according to the nature 

 of the medium of growth, changes in form and properties are 

 common. They may be slight, but if looked for are almost sure 

 to be found. I would proceed to mention cases in which such 

 changes are very marked. 



(1) Change of Form. — The Bacillus Zopfii, 3 for example, when 

 grown on various nutrient media manifests itself at first in the 

 form of long twisting tortuous filaments ; at a later period these 

 give place to chains of bacilli of fair size, and eventually in an 

 old growth the bacilli are replaced by what some would consider 

 as cocci, others as spores. We need not now concern ourselves 

 about this controversy. The important point is that, trans- 

 ferred to a fresh " soil," the bacillary form, just like the spore or 

 coccus form, develops into typical long convoluted threads. 



Even more definite changes of shape are producible by slight 



1 Nageli, Die niederen Pike, Munich, 1877. 



2 Lankester, Quart. Journ. Micr. Science, xiii., 1873, p. 408. [It may in- 

 terest Sir Ray Lankester to observe that I have been familiar with the observa- 

 tions upon his peach-coloured bacterium for a quarter of a century and more, 

 and have given him due credit. The reason why I did not refer to these in my 

 Croonian Lectures is obvious : he has consistently ascribed the modifications 

 in form to indirect and not to direct equilibration.] 



3 Kurth, Botanische Zeitung, 1883. 



