INTRODUCTION ii 



obtained by this method it does not follow that the results will 

 be invariably negative under all conditions of growth. The 

 sulphur bacteria are exceedingly plastic, and it is not to be 

 expected that all their capabilities will be manifested under 

 the artificial conditions of the glass slide and coverslip ob- 

 servations. Under the stress of competition with other organ- 

 isms in nature they sometimes exhibit a variety of forms 

 that do not appear when they are cultivated under cloistered 

 conditions. Hence if a certain phase of development is 

 observed under natural conditions the value of the discovery 

 is not nullified if the same development cannot be produced 

 under certain other artificial conditions. With such plastic 

 organisms special circumstances bring out special reactions. 

 Special emphasis is laid on this point because in the observa- 

 tions of some of the investigators, and notably of Winogradsky, 

 it seems to be assumed that that which does not appear under 

 artificial conditions cannot appear under natural conditions 

 under any circumstances whatsoever. 



3. Method of Pure Culture. — Pure cultures of the sulphur 

 bacteria have been obtained by Keil for the colourless, and by 

 Bavendamm for the coloured, bacteria. This is the most 

 valuable method of observation in the investigation of 

 physiological problems, many of which cannot be determined 

 in any other way. The same defect is, however, inherent in 

 this as in the previous method, for the course of the life- 

 history of bacteria is seldom the same under the artificial 

 conditions under which the cultivation must necessarily be 

 conducted, as it is under natural conditions. Hence whilst 

 the positive results obtained by this method are invaluable, 

 and particularly in physiological investigations, judgment 

 must be deferred if the results are negative, especially if such 

 results are in contradiction to the positive results obtained by 

 other methods. 



All three methods have contributed their share to the 

 building up of the present body of our knowledge of the 

 sulphur bacteria. 



