78 Treatment of Grass Land for Elimination of Disease. 



recognisable disease, a circumstance due tu the fact that the 

 seeds are not available in numbers sufficient to induce 

 appreciable symptoms, or that conditions in the outer world are 

 inimical to their existence. This, perhaps, more generally 

 applies to parasitic worms, for a striking difference between 

 these and bacteria as disease-producing agents is that 

 while the latter, after gaining the animal body, multiply so 

 enormously that a minute dose may set up disease in fatal 

 form, the development \vithin the animal, of ova or embryos 

 of worms produced there, to a stage at which they become 

 injurious to the host does not usually occur. In case of neither 

 is this rule absolute, for, small as the dose may appear, it is 

 known that the effects of some bacteria depend materially on 

 the number introduced into an animal in a given time ; while 

 in the case of certain parasitic disease the principal mischief is 

 produced in the animal by embryos expelled by mature worms 

 within it. Inasmuch, therefore, as many apparently healthy 

 animals are discharging germs, which in large numbers and 

 under certain conditions maj* prove the cause of disease, it 

 stands to reason that the thicker animals are on grass land the 

 greater will be the risk of serious contamination, and in this 

 lies the explanation of some of the ills of overstocking. Long 

 before the existence of these conditions was generallj^ realised, 

 common observation had told our forefathers that frequent 

 change and ample grazing space were essential to the health of 

 live stock, particularly of sheep ; and the value of acting on 

 their maxim is thoroughly supported by the revelations of 

 modern research and experience. 



Important, however, as is recognition of the fact that these 

 diseases of grazing animals are caused by living organisms and 

 that if these were not accessible to our live stock our pastures 

 would, in this respect, be free from danger, it must be realised 

 that conditions in the outer world favourable to their develop- 

 ment are of extreme moment. Fortunately, only an 

 infinitesimal proportion of disease germs cast forth on to 

 grass land survive under conditions ordinarily obtaining in 

 nature, but it is too well known that abnormalities of season 

 and other circumstances, often beyond our control, may 

 provide conditions under which they flourish and pollut 

 grass land to a dangerous degree. Indeed, while it is impos- 

 sible to determine whether any given animals are or are 

 not discharging disease germs, it would appear that the 

 means at our disposal are insufficient to ensure complete 

 security against occasional dangerous contamination. It wall 

 be realised that the seeds of disease may be brought by animals 

 into situations in which the disease for which they are respon- 

 sible had not been previously known to exist, and, if favourable 



