38 THE ANATOMY OF LNVERTEBRATED ANIMALS. 



that the general belief was erroneous in a multitude of in- 

 stances ; tSpallanzani added largely to the list ; while the in- 

 vestigations of the scientific helminthologists of the present 

 century have eliminated a further category of cases in which 

 it was possible to doubt the applicability of the rule " omne 

 vivum e vivo " to the more complex organisms which consti- 

 tute the present fauna and flora of the earth. Even the most 

 extravagant supporters of abiogenesis at the present day do 

 not pretend that organisms of higher rank than the lowest 

 Fungi and Protozoa are produced otherwise than by genera- 

 tion from preexisting organisms. But it is pretended that 

 Bacteria, Torulas, certain Fungi, and "Monads," are de- 

 veloped under conditions which render it impossible that 

 these organisms should have proceeded directly from living 

 matter. 



The experimental evidence adduced in favor of this prop- 

 osition is always of one kind, and the reasoning on which 

 the conclusion that abiogenesis occurs is based may be stated 

 in the following form : 



All living matter is killed by being heated to n degrees. 



The contents of a vessel, the entry of germs from without 

 into which is prevented, have been heated to n degrees. 



Therefore, all living matter which may have existed there- 

 in has been killed. 



But living Bacteria, etc., have appeared in these contents 

 subsequently to their being heated. 



Therefore, they have been formed abiogeneticallv. 



No objection can be taken to the logical form of this rea- 

 soning, but it is obvious that its applicability to any particu- 

 lar case depends entirely upon the validity, in that case, -of 

 the first and second propositions. 



Suppose a fluid to be full of Bacteria in active motion, 

 what evidence have we that they are killed w T hen that fluid 

 is heated to n degrees ? There is but one kind of conclusive 

 evidence, namely, that from that time forth no living Bacteria 

 make their appearance in the liquid, supposing it to be prop- 

 erly protected from the intrusion of fresh Bacteria. The 

 only other evidence, that, for example, which may be fur- 

 nished by the cessation of the motion of the Bacteria, and 

 such slight changes as our microscopes permit us to observe 

 in their optical characters, is simply presumptive evidence of 

 death, and no more conclusive than the stillness and paleness 

 of a man in a swoon are proof that he is dead. And the 

 caution is the more necessary in the case of Bacteria, since 



