30 ADAPTATION AND DISEASE 



ing it from the ordinary B. coli and with fermentative powers 

 more active than normal. If we accept his argument, here is 

 a typical example of a true mutant, for the development of 

 which the only satisfactory explanation is, that there has been 

 some accidental protoplasmic inequality in some one cell division, 

 and that one of the two resultant cells has by this inequality 

 gained in excess or lost some one protoplasmic constituent ; 

 thereby a new equilibrium has had to be established between 

 the organism and its surroundings, this adaptation showing 

 itself in some alteration of properties. 



I fancy that we all look askance at any invocation of 

 " accident " to explain natural phenomena, regarding this as 

 a negation of natural law, as something reprehensible, evasive, 

 and almost dishonest, in that the more we study nature the 

 more it is impressed upon us that there is a cause for everything. 

 In this very instance it may be pointed out that while all organ- 

 isms floating in a fluid medium appear to be subjected to the 

 same influences, as a matter of fact they are not. In such 

 medium organisms at the surface are exposed to an environ- 

 ment which differs widely from that of organisms totally sub- 

 merged, and the analogy may be drawn between what occurs 

 here and the tendency exhibited by many yeasts to exhibit 

 septal, filamentous forms on surface growths, and isolated torula 

 forms when submerged. 



THAT IN THE BACTEBIA MUTATIONS ARE IMPRESSED VARIATIONS 



So far as I can determine the majority of instances of bacterial 

 mutation depend or follow upon a preliminary alteration of 

 environment, and these alterations must be regarded as the 

 controlling factors, resulting in impressed variations. Numerous 

 cases are on record in which, by growing bacteria in media 

 containing certain sugars, eventually they have gained the 

 power to split up and ferment these sugars, or in media, like 

 milk, containing fats, have developed lipolytic powers ; have, 

 that is, adapted themselves to feed upon the new food-stuff. 

 Close upon thirty years ago the late Sir Lauder Brunton and 

 Macfadyean l reported experiments of this nature, showing that 

 by growth in the presence of particular sugars certain species 



1 Proceedings of the Royal Society, xlvi., 1889, 542. 



