BACTERIAL CLASSIFICATION 15 



character — not the individual organism — but the 

 aggregate of the correlations of various properties as 

 manifested in a considerable series of individuals, cer- 

 tain well-defined systematic units appear, marked by 

 the general association of a number of independent 

 characteristics. Such an association can be explained 

 only on the ground of, relationship, and the types marked 

 by the simultaneous occurrence of a number of properties 

 may rightly be taken as the major centers from which 

 other more aberrant individuals have varied. 



It must be remembered in discussing the occurrence 

 of correlated characters that their common presence 

 may be due to one of two causes. The characters may 

 be essentially independent ; and may be correlated simply 

 because ancestral forms evolved them under the action 

 of wholly different causes. Such correlations are obvi- 

 ously of the highest phylogenetic importance. On the 

 other hand, correlation may be due to the fact that the 

 properties studied are not really independent but are 

 so bound up together as to vary simultaneously. The 

 selective action of the environment may produce a 

 parallel change in both; or the two characters may be 

 so related in the physiological balance of the organism 

 that a change in one leads to a corresponding modifica- 

 tion of the other. 



Types marked by the correlation of truly independent 

 characters might naturally be expected to be most signifi- 

 cant, — they correspond more closely to the modifications 



