INTRODUCTOEY NOTE. xi 



its present geographical limits. Whether there is any 

 evidence of its modem or recent introduction into 

 countries previously exempt. How far any such disease 

 may have been prevented from invading new countries, 

 or from spreading from any particular centre, by 

 measures directed against contagion. Above all, to 

 determine what is the nature, and what the true value, 

 of the evidence supposed to show that the specific 

 poison of a contagious disease may originate spon- 

 taneously, or be generated de novo. ' What we most 

 want to know,' adds Budd, 'in regard to this whole 

 group of diseases is, where, and how, the specific 

 poisons which cause them, breed and multiply.' 



Budd's own relation to the question here raised 

 was distinct and, under the circumstances, impressive. 

 ' After giving many years of time and thought to an 

 examination of the evidence bearing on this question,' 

 he comes to the conclusion 'that there is no proof 

 whatever' that the poisons of specific contagious 

 diseases ever originate spontaneously. ' That the 

 evidence on which the contrary conclusion is founded 

 is negative only ; that evidence of precisely the same 

 order, only to all appearailce still more cogent, would 

 prove animals and plants, even of large species, to 

 originate spontaneously; that this evidence is therefore 

 of no weight ; and, lastly, that aU the really important 

 facts point the other way, and tend to prove that these 

 poisons (to use a term which is probably provisional 

 only), like animals and plants, however they may have 

 once originated, are only propagated now by the law of 

 continuous succession.' 



