38 



cellularity of the microbes, it is my opinion that by it the phenomena of variation are 

 rendered clearer but are not changed, when compared to the multi-cellular organisms. 

 According to the point of view, the individual microbe can be compared to the whole 

 individual of the higher organism, or to a single tissue-cell oft it, - - both comparisons 

 are correct J ). 



i. Degeneration. 



In bacteriological laboratories it is well-know that by prolonged culture many 

 microbes undergo slow, but great changes, even in so much, that certain long continued 

 cultures do not agree any more with the descriptions given of them by the discoverers, 

 short after their first isolation from nature. In some cases the way in which the 

 change takes place can be rather minutely traced; three forms of variability are 

 therein more salient: degeneration, transformation and common variation. 



A species is isolated from nature and it is found that at the culture during the 

 first series of inoculations, in which hundreds or thousands of cell-generations succeed 

 each other, it develops well, so that m the beginning the impression is obtained of a 

 thorough knowledge of the nutrition and other conditions of life. But by and by it 

 becomes more difficult to make the new inoculations thrive and at last the culture- 

 material grows troublesome and uninteresting and would be quite unrecognisable if 

 not the various phases of the degeneration-process had been exactly observed. Pro- 

 longed cultivation above the optimum temperature of the growth, and a too strong 

 concentration of the nutriment are in some cases the cause of degeneration. In some 

 microaerophilae, for instance the bactery of the lange-wei (Streptococcus hollaii- 

 diae) 2 ), the irrational regulation of the oxygen tension causes a rapid, in few days 

 complete vanishing of the slimeformation, while after a much longer time, by the same 

 cause, the vegetative power of the bactery completely disappears. In other cases, for 

 instance with a phosphorescent bactery, very common in the sea (Photobacter 

 degenerans Fischer), the degeneration is accomplished without known cause, and 

 in a very short time, so that, within a few weeks the cultures may cease to exist. The 

 degeneration goes not by leaps but continously and affects all the individuals in cul- 

 ture equally, so that it cannot be checked by colony-selection. 



'2. Transformation. 



At the transformation all the individuals brought in culture lose a characteristic, 

 while either another comes in its place, or a new characteristic arises, or, lastly, the 

 characteristic disappears without a distinct substitute. Thus the cultures of Photo- 

 bacter luminosum grow dark in the course of some months by a slow process of trans- 

 formation, whereby they change into a more rapidly growing form, which acts more 

 strongly on the nutriment than the normal form. Here, thus increase of vegetative 

 power has supplied the decrease of phosphorescence. 



') An interesting view herewith connected is found in Whitman, The inadequacy 

 of the cell-theory of development. Biological Lectures at the Wood's Holl Laboratory, 

 1893, pag. 105, Boston 1894. 



-) Used in Holland for cheese-making. 



