POKM AND CLASSIFICATION OF BACTERIA. 23 



kind than others. Variations in the form and size of the 

 individuals of a given species must be expected. The same 

 form will be maintained only when all the conditions of 

 environment are constant. 



In view of the marked alterations in the form of bac- 

 teria, it has been supposed that there is a corresponding 

 variability of species. Under certain conditions of environ- 

 ment a given species may cease to produce the pigment 

 which under normal conditions it would elaborate. Again, 

 similar unfavorable conditions may cause a disease-pro- 

 ducing germ to lose its property of growing in the living 

 body. In other words, not only the mere form and size, 

 but also the physiological activities of the cell may be 

 altered. This weakening of the functional activity of a 

 bacterial cell is known as attenuation. The attenuated or 

 weakened germ constitutes a variety of the original type, 

 but it does not constitute a new species. On reversing the 

 conditions of environment, that is rendering these more 

 favorable, the attenuated form can be brought back to the 

 full possession of its original properties. 



Undoubtedly, species have originated in the past as a 

 result of growth under altered conditions, and new species 

 may even now be in process of formation. But under 

 ordinary -conditions of observation this change of one - 

 species into another does not and cannot take place. The 

 hay bacillus cannot be transformed into the anthrax bacil- 

 lus, nor can the germ of pneumonia be converted into that 

 of consumption. They may both be greatly modified, 

 giving rise to varieties, but so long as these exist they still 

 represent the original species. The typical form and the 

 typical species can always be reproduced by restoring the 

 most favorable conditions of growth. 



