ORIGIN OF DISEASE GERMS. 251 



living, without being indebted for existence to a 

 previous generation. 



On the Origin of Disease Germs. We have now to 

 try to find an answer to some very important questions, 

 among which are the following: Have disease germs 

 always existed ? Is their origin contemporaneous 

 with that of life, or is it more probable they are of 

 more recent date ? Could they have existed before 

 man, and independently of man, or is their production 

 necessarily associated with human beings ? If the 

 latter appears to be the most probable view, we may 

 enquire further whether there were contagious disease 

 germs in the prehistoric epoch, or if their evolution is 

 in some manner dependent upon civilization. The 

 thoughtful student naturally desires to know if disease 

 germs of each special kind known to us at this time, 

 always retained the characters they now possess, or if 

 their properties have become modified from time to 

 time as external circumstances have changed. Are 

 disease germs ever developed anew in these days ? 

 Do generations of disease germs die out and give place 

 to others allied to them, but not identical ? And, 

 lastly, the important practical question which has been 

 already referred to in page 249 presents itself. Sup- 

 posing it were possible to extirpate entirely all disease 

 germs now existing, is there reason to think that new 

 forms of precisely the same kind would be reproduced, 

 or that more or less closely allied forms would be de- 

 veloped anew ? 



