164 HETEROGENETIC ORIGIN OF 



into a host plant is first to kill the cell by the emission of the 

 toxin ; the germ-tube then penetrates the dead cell and is 

 nourished saprophytically upon it ; with the vigour thus gained 

 it destroys the neighbouring cells and passes from one to another 

 without further difficulty. The fungus-hypha has the power of 

 penetrating the cuticle, but only in young and tender structures ; old 

 and hardened membranes could only be entered when the cuticle 

 had been injured, or when it [the hypha] had gained strength by 

 special saprophytic nutrition." 



These results will prove important as standards for comparison 

 with other observations of my own which I am about to cite. We 

 see that in the case of actual infection by Bacteria there is need 

 for the co-operation of many full-grown active organisms (and not 

 mere motionless germs) in order to bring about, by their secretions, 

 the softening of the wall of every single cell that is penetrated ; 

 that some time is required for the operation ; and that the soften- 

 ing produced must be considerable before any such penetration 

 is possible by the moving organisms. It will be seen how very 

 different is the state of things, in cases which I shall cite as 

 instances of the origin of Bacteria and their allies, in the tissues 

 of plants and animals, by a process of Heterogenesis. 



It will be found, in fact, that the presence of two characteristics, 

 wherever they co-exist, may be regarded as strongly in favour 

 of the interpretation of Heterogenesis as against Infection, as the 

 following remarks will show. 



(i) The means adopted by Bacteria for bringing about the 

 penetration of cells being such as are associated with the vital 

 processes of adult organisms^ there is no reason to think that invisible 

 or scarcely visible germs of such minute organisms would have the 

 power of secreting a cytase sufficient in amount to bring about 

 the degree of softening of a cell-wall which has been found to be 

 a necessary preliminary to their penetration. Yet in multitudes of 

 cases it is minute germs of Bacteria and their allies which may be seen 

 developing within cells or tissues. 



(2) Again, the process of infection, as described by Potter, is 

 one brought about by active organisms which affix themselves to 

 a cell-wall until it becomes softened, and then succeed, by reason 

 of this same activity, in boring their way into the cavity of the cell. 

 On the other hand in very many of the cases in which, as I main- 

 tain, Bacteria and their allies may be presumed to be originating 

 by Heterogenesis, what can often be seen is this — particles 



