FORM AND CLASSIFICATION OF BACTERIA. 19 



be observed in the form and size of various species. Thus, 

 one species may be very short and thick, while another 

 may be considerably longer and narrower, or longer and 

 thicker. Again, in one species of bacillus the ends may be 

 square, while in others they may be rounded or ellipsoidal. 

 Among the spirilla marked variations in form or size will 

 be observed. So much so that it is very doubtful at times- 

 whether certain species really belong to the group of bac- 

 teria. 



While some species of bacteria show marked differences 

 in form and size, it must not be supposed that this is always 

 the case. On the contrary many undoubtedly distinct 

 species of bacteria may have the same form and size, so 

 that, as far as the microscope is concerned, they cannot be 

 distinguished one from another. The division of micro- 

 cocci, bacilli and spirilla into species is based, as a rule, 

 not upon the mere microscopical appearance, but rather 

 upon the sum total of the properties they possess. The 

 characteristics of growth on various artificial media, the 

 behavior to staining reagents, and the effect on the living 

 animal must all be considered when an attempt is made to 

 to distinguish one species from another. For this reason it 

 is not possible, except in very few instances, to identify 

 species among bacteria by mere microscopic examination. 

 In the majority of cases the identification of species neces- 

 sitates a careful, long and tedious study of all the proper- 

 ties possessed by that organism. 



The higher forms of plant and animal life possess, as a 

 rule, a constant form and size. Marked changes in the 

 environment, such as temperature, and altitude, are neces- 

 sary to produce type variations. Bacteria, however, are 

 extremely liable to undergo alterations in form, size and 

 other characteristics. Owing to their simple nature they 

 are readily influenced, in some way or another, by the 

 slightest change in their environment. A variation of a 

 few degrees in temperature, a trifling difference in the 



