2 TEXT-BOOK OF BACTERIOLOGY. 



a preliminary work, and that it belonged to later investigators to 

 show whether or not his form-classes and form-species would agree 

 with proper natural-history classes and species. He was, however, 

 misunderstood, and people objected to what he called "the con- 

 stancy of forms." The following will show the views on this sub- 

 ject held by observers at that time: "Within the last ten years," 

 says Nageli, " I have examined thousands of dividing yeast forms, 

 and I should be unable to maintain (if I except sarcinae) that there 

 is a need even of a division into two specific forms." And again, 

 there is that often-quoted expression, in which he draws the logical 

 conclusions of his investigation : " If my view is correct, the same 

 species in the course of generations take different morphological 

 and physiological forms, which, in the course of years, bring about 

 sometimes the sourness of milk, or the acid formation of sauer- 

 kraut, or the fermentation of wine, or the putrefaction of albuminous 

 substances, or the reddening of foods containing starch; sometimes 

 producing tj^phus fever, cholera, or intermittent fever." 



It is a matter of course that if the above were correct, scien- 

 tific investigation of bacteria would be an impossibility. Yet at 

 the present day no one who has seriously occupied himself with 

 these matters, doubts that there is a long series of species clearly 

 differing one from the other under all circumstances, both in their 

 physiological and morphological character. 



We know that certain species of bacteria, at all times and under 

 whatever conditions they are placed, in their development show 

 the same expressions of life which in essentials always remain in 

 harmony, thereby distinguishing them from other bacteria. We 

 also know bacteria which, under the most varied conditions, always 

 possess the same form, and thus are always distinguishable from 

 others. Practice alone enables us to recognize these fine differences 

 in form. Not only the ultimate varieties, but also variations in the 

 same species, and even within the same cell differences are percepti- 

 ble on examination. 



Let us now consider the different conditions to which we must 

 submit the bacteria in preparing them for examination. If we ex- 

 amine anthrax bacilli from a young gelatin culture with a high 

 power (-fiy), we shall see a number of separate motionless and color- 

 less bacteria, which appear as long rods, apparently uniform. 



If bacilli from the same source be treated with coloring agents 

 (gentian-violet), the rods will appear thicker and shorter. The col- 

 oring matter has entered them, has formed a layer upon them, has 

 so to speak formed a mantle around them, and hence the ap- 

 parent enlargement. If we examine a properly-stained prepara- 



