THE DOCTRINE OF CONTAGIUM VIVUM. 313 



in decomposing jluids owe their origin exclusively to parent 

 germs derived from the surrounding media. 



But how, you will ask, has it been possible, in the face of 

 this evidence, to maintain, with a show of success, the con- 

 trary opinion that bacteria can and do, exceptionally at least, 

 and in certain media, arise spontaneously ? This opinion is 

 based on two undoubted facts, which, taken together, seem 

 at first sight to stand in direct contradiction with the pro- 

 positions 1 have enunciated. The first fact is that bacteria 

 are invariably killed when exposed to a temperature of about 

 140 deg. F., or any higher temperature. The other fact is 

 that certain liquids, such as neutralised hay-infusion and 

 milk, often produce bacteria after having been boiled, some- 

 times after being boiled for two or three hours, and when 

 there was no possibility of subsequent infection. It seemed 

 at first sight a fair inference from these two facts that the 

 apparition of organisms in boiled liquids was due to spon- 

 taneous generation, or abiogenesis. It does seem difficult to 

 believe that any living thing can survive a boiling heat for 

 several hours, and yet such is undoubtedly the truth. When 

 I published on this question in 1874, I advanced more than 

 one line of proof which appeared conclusive that germinal par- 

 ticles of some sort did, under certain circumstances, survive 

 a boiling heat ; and that the instances referred to were ex- 

 amples of such survival, and not of a de novo generation. 

 But I was not then able to explain the apparent contradic- 

 tion involved in these experiments. 



Since then, a new and surprising light has been thrown on 

 this subject by the researches of Professor Cohn of Breslau, 

 and we are now in a position to offer a complete solution of 

 the riddle. All the confusion has arisen from our having 

 failed to distinguish between the growing organism and its 

 seed or spore. You are all familiar with the immense dif- 

 ference in vital endurance between the seed and the growing 

 plant. The same difference exists between a spore and its 

 offspring. Some spores have an extraordinary power of re- 

 sisting heat. Mr. Dallinger and' Dr. Drysdale, in the course 

 of their inquiries into the life-history of septic monads, 

 demonstrated that while the living monads are killed by a 

 heat of 140 deg. F., the spores of one variety, which are so 

 minute that they cannot be seen, except in mass, by the 

 highest powers of the microscope, are capable of germinat- 

 ing after being subjected to a heat of 300 deg. F. for ten 

 minutes ! If the spores of monads can resist this tremen- 

 dous heat, there is no reason why the spores of bacteria 

 should not be able to survive the feebler heat of boiling water. 



