VARIABILITY OF SPECIES. 117 



first asserted this in a broad sense, was partially led to this 

 conclusion by inefficient methods. But also Cohn's doc- 

 trine of the constancy of species, which for a long time 

 was most strongly advocated by Koch and his pupils, is 

 not to-day tenable in the old sense, for continued and 

 always more penetrating investigation has demonstrated 

 that almost all the properties of a well-defined spe= 

 cies are exceedingly variable. For example, we have 

 learned that upon various nutrient media the microscopic 

 forms vary throughout a wide range; that dwarf forms 

 occur; that the liquefaction of gelatin (p. 61) and for- 

 mation of pigment (p. 69), clouding of bouillon, pellicle 

 and sediment production, ability to produce fermentation 

 (p. 86) and pathogenic effects (p. 94) are exceedingly 

 variable quantities, which can vary from a maximum to 

 nil; even the ability of forming spores (p. 26) and, appa- 

 rently, the production of flagella and spontaneous motility 

 (p. 24) are properties that may be lost, although rarely. 

 This means that bacteria vary as remarkably as other 

 known plants, somewhat similarly to many cultivated 

 plants. 



For many of these variations one may recognize the 

 cause in the influence of the nutrient medium, and speak 

 of them as adaptations to changed conditions of life, as 

 variations from external causes. Other observations, of 

 which we related a great many in the first edition (the 

 origin of organisms obtained upon plating a culture, which 

 are entirely different as regards liquefaction and color, 

 while the original culture had for many generations ap- 

 peared pure), can properly be explained as dependent upon 

 internal causes. 



While we may deplore these facts from a didactic stand- 

 point, since they make the teaching and learning of bacte- 

 riologic science much more difficult, and not rarely also 

 made the solution of a concrete problem by the expert 

 impossible, still we must not overlook them if we would 

 advance scientific bacteriology. It is possible that the 

 hope of those may be realized who expect that new inves- 

 tigations may disclose hitherto hidden diagnostic aids, 

 which, consequently, when applied may disclose the 

 longed-for constancy and sharp definition of species. Un- 



