118 ON VARIABILITY AND ADAPTATION 



few exceptions were colourless, and though with further succes- 

 sive cultivations there was manifested a tendency for many of 

 the latter colonies to revert to type and develop red pigment, 

 yet by careful selection he was able to gain an absolutely de- 

 colourized race which remained colourless, that is, upon agar- 

 agar, and in broth at a temperature of 25° to 35° C, a temperature 

 and media at which and in which the original growth always 

 shows a strong reddish-violet tint. On potatoes at the same 

 temperature he was able to make thirty-two successive cultures 

 extending over a year, and in these not the least trace of colora- 

 tion ever appeared. 



These facts, if alone they were all that we had to consider, 

 would fully justify the statement that it is possible to evolve 

 artificially species or subspecies of bacteria. But there are 

 other facts which must of necessity be taken into account before 

 we can arrive at any definite conclusion — facts which so far as 

 I know apply at present to micro-organisms alone, though every- 

 thing points to the belief that they are operative, and should be 

 considered, in discussing the development of new species and 

 subspecies, among higher organisms. 



In cultivating the bacteria we are confronted with the fact 

 that successful culture necessitates the employment of special 

 media of growth. We have to sow the microbes in fluid or solid 

 substances, whose composition depends upon the mode of life 

 of the special form studied. And thus there is and can be no 

 common soil in which all forms propagate themselves with equal 

 facility. We are deprived of what I may term a common base. 

 We cannot, with certainty, say that such and such a form, 

 which we have separated out, is a distinct species, inasmuch as, 

 grown upon a common normal soil, it presents permanent char- 

 acteristics. The most that we can do is to approach, as near as 

 possible, to the formation of such a common soil. Thus for 

 many pathogenic and most other bacteria we find that slightly 

 alkaline beef-broth, and beef-broth that has been rendered solid 

 by the addition of gelatine or agar-agar, are media in which 

 growth occurs freely, and in which, certain elementary precau- 

 tions being taken, a given micro-organism retains its characters 

 for countless " generations," retains them so completely that 

 in the case of the majority of pathogenic microbes a series of 

 successive cultures, if made with due precautions and sufficient 



