SEED AND SOIL 27 



(B) lies quiescent, producing no apparent disease. The cause of this 

 extraordinary fact may be a question of different virulence in the 

 two bacilli, but is much more likely to be due to the greater vigour 

 and power of resistance of the mucous membrane of B's throat. 

 Sewer air, as we shall see subsequently, does not contain many 

 bacteria, and probably does not frequently convey germs of disease. 

 But this does not prove that the inhalation of sewer air will not 

 weaken the throat, and so form a favourable nidus for organisms 

 restiag there, or organisms shortly to be inhaled from dust or mucous 

 particles from the throat of a diseased person. Which is the more 

 important preventive method, to maintain the resistance of the 

 individual or to waylay the infecting organism, is a nice point we need 

 not attempt to decide. Obviously, both objects should be kept in 

 view. Phthisis is another example. Thousands of persons inhale 

 the tubercle bacillus who are not attacked by the clinical disease of 

 consumption. This fortunate result is due to the resistant tissues 

 of the healthy lung, and the lesson to be derived is to maintain such 

 resistance at its maximum. This evidently is, in part, the scientific 

 explanation of Koch's dictum, " It is the overcrowded dwellings of 

 the poor that we have to regard as the real breeding places of 

 tuberculosis ; it is out of them that the disease always crops up anew, 

 and it is to the abolition of these conditions that we must first and 

 foremost direct our attention if we wish to attack the evil at its root 

 and to wage war against it with effective weapons."* Part of the 

 explanation of these words is doubtless that it is in such places that 

 the tubercle bacillus breeds and passes from one person to another. 

 But every sanitarian knows that the effect of such environment is 

 to lower the natural resistance, to weaken the lung, impoverish the 

 blood, and undermine the constitution, and thus a suitable nidus is 

 supplied to the invading bacillus. "A perfectly healthy lung is 

 seldom if ever primarily infected with the tubercle bacillus " (Wood- 

 head). 



But the evidence of bacteriology as to the part played by the soil 

 is even stronger than at first sight appears. Por we now know, by 

 experiment, that micro-organisms which in some animals produce 

 acute disease rapidly ending in death, result only in mUd disease in 

 other animals, and in yet a third group produce no apparent disease 

 whatever. This is not due to variation in virulence but to variation 

 in soil. 



The advance of bacteriology has been so rapid and marked by 

 such striking discoveries that there has been a tendency to over -rate 

 altogether the potentiality of the bacillus apart from its medium. 

 The latest findings in the study of comparative culture work, of 

 immunity and of the production of antitoxins have, however, demon- 

 * Trans. Brit. Cong, of Tuberculosa, 1901, vol. i., p. 31. 



