162 HETEROGENETIC ORIGIN OF 



produced therein by Heterogenesis (possibly in the fluids by Arche- 

 biosis), or is their presence invariably to be ascribed to ' infection ' 

 from without ? 



(a) During the last thirty years it has been commonly held in 

 accordance with the teachings of Pasteur, Lister and others that 

 the tissues and fluids of healthy animals and plants are germless, 

 and altogether free from microorganisms. 



In regard to animals, however, it is clear that this position is one 

 which cannot be accepted without very important limitations. It is 

 obvious that microorganisms may, like other particles, get taken up 

 from the mucous membranes of the alimentary canal and the 

 respiratory system, and pass by means of lymphatics into the 

 nearest glands. If they should get through these and ultimately 

 pass into the blood, the generally accepted view is that they are 

 speedily " destroyed " in this fluid. This view is based upon the 

 fact that bacteriologists are never able to get evidence of the 

 existence of microorganisms, or their germs, by the inoculation of 

 different suitable and sterilised media, with blood drawn from a 

 healthy man or from one of the lower animals similarly free from 

 disease. Nothing more need be said now, as we shall have to 

 return to this subject later on. 



In regard to plants, that is, fruits and vegetables of different 

 kinds, the case is not so complicated, and Pasteur was probably 

 quite right in declaring that, when healthy, their cells and 

 tissues are germless. Thus, in considering the interpretation of 

 cases in which microorganisms are found in the interior of certain 

 vegetables or fruits after they have been submitted to various 

 unnatural conditions, the question will not be whether we have had 

 to do with the wakening up of latent pre-existing germs, but 

 rather, whether the organisms found are results of an infection that 

 has recently been brought about — that is, during the exposure of 

 the vegetables or fruits to the experimental conditions. And this 

 brings us to the consideration of the second of the two possibilities 

 above referred to. 



(b) The second possible mode of accounting for the presence of 

 microorganisms in the tissues of healthy animals and plants, other 

 than by Heterogenesis, is that they have resulted from some 

 process of infection brought about antecedently to, or during 

 the continuance of, the experimental conditions to which they have 

 been subjected. 



