THE BEGINNINGS OF LIFE. 415 



In the great majority of cases oxygen may be the 

 initiator of the molecular change which the fer- 

 mentable substance or the ferment undergoes; it 

 would seem scarcely probable, however, that in the 

 absence of free oxygen, no other conditions would be 

 adequate to disturb the delicate balance existing be- 

 tween the elements of a highly unstable substance. 



In considering such a subject it is of great import- 

 ance always to bear in mind the various degrees of 

 molecular mobility of different substances, and also the 

 fact that some substances will easily decompose under 

 the influence of conditions which do not affect other 

 compounds of equal complexity. Individual differences 

 or peculiarities cannot be ignored. Under the influence 

 of any particular set of conditions, therefore, organic 

 substances may be ranged under two distinct categories, 

 with respect to their degree of fermentability. Substances 

 which are to be placed in the frst class are so unstable 

 that they decompose c spontaneously ' and without the 

 aid of a separate ferment j whilst those which possess 

 only the second degree of fermentability cannot by 

 themselves be made to initiate a fermentative change 

 require to be brought into contact with a more 

 unstable substance whose motor-decay may impart the 

 needful molecular movement. Once initiated, the 

 process of change is afterwards easily maintained, even 

 in those bodies which possess only the second degree of 

 fermentability. This distinction is one of a most 

 important nature, and will subsequently help us to 



