THE STRUCTURE OF BACTERIA. 23 



that individual forms are at times tolerably 

 constant, but frequently change somewhat 

 under changing external conditions. The ex- 

 istence of rigid form-species, which not only 

 the earlier observers, but even Cohn, Schroter 

 and Koch assumed, can be upheld no longer. 

 The adaptability of bacterial forms to changing 

 conditions of nutrition is not so boundless as 

 Naegeli and Billroth supposed, but it is con- 

 siderably greater than was once held to be com- 

 patible with the conception of the existence of 

 constant species. 



In the first instance form-species or form- 

 genera were recognized and these were named 

 after the most prominent forms. It was in 

 accordance with this tendency that Ferdinand 

 Cohn, turning the older names to good account, 

 applied the generic title Micrococcus to the coc- 

 cus form, Bacterium to the short rods, Bacillus 

 to the long rods, and Vibrio, SpirocJuete and 

 Spirillum to the various spiral forms. These 

 designations did not really serve to distinguish 

 a single species, and Naegeli and Koch added 

 more confusion to a matter which was in itself 

 simple. The name Bacillus was applied by 

 these investigators not to long rods but to a 

 genus or species whose most" essential " form 

 was a long rod. What else characterized it, 

 whether it changed its form or whether it pro- 



