THE CHAEACTEEISTICS OF BACTERIA 107 



protozoa we must look for the origin of " hereditary individual 

 variability," and, dealing with them, emphasizes this influence of 

 environment. If, for instance (he states), a protozoon, by con- 

 stantly struggling against the mechanical influence of currents in 

 water, were to gain a somewhat denser and more resistant proto- 

 plasm, or were to acquire the power of adhering more strongly 

 than other individuals of its species, the peculiarity in question 

 would be directly continued on into its two descendants, for the 

 latter are at first nothing more than the two halves of the 

 former. It therefore follows that every modification which 

 appears in the course of its life, every individual character, how- 

 ever it may have arisen, must necessarily be directly transmitted 

 to the two offspring of a unicellular organism. 1 



And in connexion with the bacteria we can modify the 

 environment at will to an extent that is far beyond our power 

 in regard to higher organisms ; we can alter the media of growth, 

 can make these more or less nutritious, can make them acid, 

 neutral, or alkaline ; can alter the superincumbent air to such 

 an extent that if desired we can in many cases bring about 

 growth in vacuo or in an inert atmosphere of hydrogen or carbonic 

 acid ; we can add or remove this or that normal or abnormal 

 food-stuff to or from the medium of growth, or can add salts 

 and antiseptics, and observe the effects upon the microbes 

 studied ; we can observe the mutual effects of concurrent growth 

 of two or more forms, and the effects which the product of 

 growth of a microbe exert upon it, or again can modify the 

 temperature within the widest limits. With no other forms of 

 life are such extraordinarily diverse modifications of environ- 

 ment possible. 



I propose now to set forth in regular order the facts at our 

 disposal with regard to the production and existence of varia- 

 tions of microbes. To give all is, from their very extent, im- 

 possible in this connexion, but sufficient may be given to show 

 very clearly that this variation is far more widespread than is 

 generally supposed, and that we can safely attribute to this 

 variability many of the difficulties already mentioned met with 

 in the clinical study of infective disease. 



It is unnecessary for me to enter into the earlier history of 

 the subject, into the controversy that raged around "poly- 

 1 Essays upon Heredity, Oxford, 1889, p. 278. 



