THE VICTORIAN NATURALIST. 157 



solution, and it will remain at rest till some of the alcohol is taken 

 ofif, or until the fluid is diluted, so as to reduce the percentage of 

 alcohol. Now, we know that many organisms produce chemical 

 compounds just in the same way as the yeast produces alcohol. 

 We have reason to think that they all do so. Whatever may be 

 the cause, it is evident that there must be something in their 

 surroundings which checks the rate of increase in the bacteria, 

 and those who can most successfully overcome that something 

 have the best chance of survival. 



INFLUENCE OF ENVIRONMENT ON BACTERIA. 



It has long been known that the nutrient medium and the 

 temperature at which bacteria are grown have a marked influence 

 on the appearances of the growing colonies. Thus an organism 

 which will produce abundance of pigment when grown on a 

 potato may produce little or none on gelatine. Some are 

 exceedingly difficult to cultivate upon any artificial medium, of 

 which a notable example is found in tubercle. It is to this 

 organism that I wish to call attention in the first place. Most of 

 you are, doubtless, aware that the tubercle bacillus is a short,, 

 thin rod, about one-third of the diameter of a red blood corpuscle 

 in length. For a long time no artificial culture medium could be 

 found till Koch hit upon solid blood serum. Subsequently 

 nutrient agar-agar, or broth, to which 6 per cent, of glycerine 

 has been added, has become the usual medium on which it is 

 grown. It flourishes best at about blood heat ; indeed, until 

 lately, it was supposed to grow only at that temperature, and 

 does not flourish vigorously, compared to other organisms, even 

 at that. If, however, the temperature be lowered, and after a. 

 few weeks' incubation a second tube be inoculated from the 

 growth which has taken place in the first one, this second crop 

 will be more vigorous than the first, and a third, fourth, and 

 subsequent generations will each display additional vigour, even 

 if the temperature be gradually reduced. Similarly, a change 

 may be produced by varying the medium, until at last a very free 

 growth may be obtained on potato kept at the ordinary tempera- 

 ture of the air — say, 5o°-7o° F. An examination of the properties. 

 of such a growth will, however, reveal considerable changes from 

 those of the original growth. " Earlier generations of such culti- 

 vations produce, when inoculated, typical tubercular nodules 

 with extreme and characteristic rapidity, but after several genera- 

 tions of such pure cultivations have been made in these glycerine 

 media, the virulence may become distinctly diminished, although 

 the growths are as luxuriant as or even more luxuriant than ever." 

 At the same time many of the bacilli exhibit marked change of 

 form. On the other hand, cultivations which have been carried 



