22 BOARD OF AGRICULTURE. 



had reached a limit where a germ was no longer introduced 

 with every inoculation, and that where a germ was contained 

 in the drop of liquid used, the disease followed in all its in- 

 tensity, while the failures were explained on the supposition 

 that the drops of liquid used in these cases contained no 

 germs. But we had numbered our germs, and we knew 

 that every drop of a dilution of one to ten thousand contained 

 more than two hundred of these, so that this explanation 

 was no longer tenable. Fortunately we noticed that with 

 many of the birds which remained apparently healthy there 

 appeared a slight local redness and swelling at the point of 

 inoculation within a week or ten days after the operation and 

 remained until the twenty-first or twenty-second day. It 

 was not exactly what \yas expected, but these birds were 

 tested by inoculation with the strongest virus and they re- 

 sisted it in a wonderful manner. They had, in fact, acquired 

 a very complete immunity. 



The virus of fowl cholera is one of the most virulent which 

 is known to attack the lower animals, and yet we found that 

 a single germ was incapable of injuring the most susceptible 

 birds. It requires numbers, and very considerable numbers, 

 to overcome the vital resistance of the animal body. As 

 many as twenty-five germs were necessary to produce an ap- 

 parent local congestion in very susceptible birds. Increase 

 this number to seventy-five or one hundred and they were 

 able to penetrate to the interior of the body, to multiply 

 themselves to an almost infinite extent, and to set up those 

 pathological changes in various parts of the body which lead 

 to the death of the individual. 



How wonderful these facts appear to us, and what remark- 

 able avenues are opened up for future research ! Why is it 

 that a single active germ introduced into the tissues cannot 

 reproduce itself indefinitely, as it does in a flask of broth, 

 and succeed in developing the disease? What is the nature 

 of the resistance by which the living tissues are enabled to re- 

 sist such a germ and finally destroy it? How can a number 

 of germs multiply where a single one cannot? Above all, 

 how can such a local multiplication of germs confer upon the 

 tissues of the whole body the power to completely resist 

 enormous numbers of such germs for years afterwards ? It 



