THE BEGINNINGS OF LIFE. 413 



things. Observation and experiment alike are abso- 

 lutely opposed to such a limitation, and even had it not 

 already been shown to be utterly erroneous, it is a 

 doctrine which ought only to have found favour with 

 those who are professed c vitalists.' Consistent believers 

 in the physical doctrine of life could scarcely be expected 

 to do other than mistrust a doctrine which would have 

 them believe either that the molecular changes taking 

 place in living things were not essentially chemical in 

 nature, or else that they were chemical changes absolutely 

 sui generis. It would be almost impossible, indeed, to 

 frame a true and distinctive definition of fermentative 

 changes. Just as we have previously urged, that the 

 living thing differs from the not-living thing in degree 

 and not in kind, since the properties of both are de- 

 pendent upon their molecular composition and structure ; 

 so does the fermentative chemical change differ from 

 the not-fermentative chemical change merely in degree 

 though even to a less extent, because these two kinds 

 of chemical change are now actually known to merge 

 almost insensibly into one another. It is almost impos- 

 sible to say where the one ends and the other begins. 



As we have already intimated, in the opinion of 

 Gay-Lussac and also of many chemists in the pre- 

 sent day, oxygen is needed to initiate the changes 

 which the ferment undergoes. According to Baron 

 Liebig, however, all that is essential in order that 

 fermentation may occur, is that a complex substance 

 should undergo changes of a particular kind, either 



