110 ON VAEIABILITY AND ADAPTATION 



of 0-02 per cent, after forty -eight hours long straight bacilli 

 are to be made out, broader than the type, and from three to 

 five times as long, If, instead, alcohol be added (about 4 per 

 cent), at the end of twenty-four hours the change is even more 

 pronounced ; besides large bacillary forms, similar to those 

 last mentioned, there may be somewhat curved filaments from 

 ten to fifteen times as long as the type, if not longer. If 0'015 

 per cent potassium bichromate be added, then, even after fifteen 

 hours, one can make out long undulating filaments stretching 

 across the whole field of the microscope. And if the growth 

 be acted upon by O7 per cent boric acid for six days, then in 

 place of the microbacillus we see " comma " forms, S-shaped 

 forms, and close spirals ; while 0-1 per cent creasote acting for 

 some weeks causes the production of micrococci and diplococci. 



There could be no better example than this of the poly- 

 morphism induced by environment. In all these cases, however, 

 we are not dealing with permanent variations. Return to 

 " ordinary " media brings about in all a return to what may 

 be styled " type." I say what may be styled " type," inas- 

 much as the broth, potatoes, and other materials employed by 

 the bacteriologist are the common soils in and upon which can 

 be grown most bacteria, but they are not of necessity their 

 natural habitat, and when environment evidently plays so im- 

 portant a part in determining the shape of bacteria it is quite 

 possible that what we consider as typical are, in many cases, 

 not the usual (or primitive) but are acquired forms, 



(2) Changes in Pigment Production. But shape is far from 

 being the only property of microbes that is modified by environ- 

 ment, and very striking results are to be gained by a study of 

 those microbes which, in the course of their growth, produce 

 pigment the chromogenic bacteria. 1 Of these a very large 

 number lose the faculty of pigment formation in the absence of 

 oxygen, many again when grown in the absence of diffuse light, 

 and all under the action of the direct rays of the summer sun. 

 An alteration in the medium of culture has also its effect. Thus 

 the Bacillus ruber of Kiel (better known in this country as the 

 " rouge de Kiel," the name under which our attention was first 

 drawn to it by Laurent 2 ), the Micrococcus indicus, the Micro- 



1 Schottelius, Kolliker's Festschrift, Leipzig, 1887. 



2 Laurent, Annales de I'Inst. Pasteur, iv., 1890, p. 465. 



