MONERA 



which are, in a very loose way, made up into special 

 groups of molecules. The chemical products of their 

 mutual action are what we call ptomaines, which are 

 partly very virulent poisons (toxins). We have suc- 

 ceeded in producing several of these poisonous matters 

 in large quantities by artificial culture, and isolating them 

 and experimentally ascertaining their nature; as, for 

 instance, tetanin, which causes tetanus, typhotoxin, the 

 poison of typhus, etc. 



In thus declaring the action of bacteria to be purely 

 chemical and analogous to that of well-known inorganic 

 poisons, I would particularly point out that this very 

 justifiable statement is a pure hypothesis ; it is an excel- 

 lent illustration of the fact that we cannot get on in the 

 explanation of the most important natural phenomena 

 without hypotheses. We can see nothing whatever of 

 the chemical molecular structure of the plasm, even 

 under the highest power of the microscope; it lies far 

 below the limit of microscopic perception. Neverthe- 

 less, no expert scientist has the slightest doubt of its 

 existence, or that the complicated movements of the 

 sensitive atoms and the molecules and groups of mole- 

 cules they make up are the causes of the vast changes 

 which these tiny organisms effect in the tissues of the 

 human and the higher animal body. 



Moreover, the distinction of the many species of bac- 

 teria is of interest in connection with the general ques- 

 tion of the nature and constancy of a species. Whereas 

 formerly in biological classification only definite mor- 

 phological characters, or definable differences in outer 

 form or inner structure, were regarded as of any moment 

 in the distinction of species, here, in view of the vague- 

 ness or total lack of these characters, we have to look 

 mainly to the physiological properties, and these are 

 based on the chemical differences in their hypothetical 

 molecular structure. But even these are not absolutely 



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