66 The Unity of the Organism 



is undoubtedly true as a general proposition, at the same 

 time the group presents a degree of structural and func- 

 tional plasticity which surpasses, probably, anything oc- 

 curring in any other group. Under the term "pleomor- 

 phism" this multiform character of bacteria has been the 

 subject of much investigation and no little heated discussion. 

 One party, headed by the German botanist Nageli, has main- 

 tained that the organisms are not classifiable in the usual 

 sense of biological taxonomy at all that any form is suc- 

 ceptible of becoming almost any other form, depending on 

 the external conditions under which it is placed. The other 

 party, of whom Cohn seems to have been the originator, has 

 stoutly insisted that species and genera are as definite in 

 bacteria as in any other group, and that every infectious 

 disease has its specific germ agent. The evidence and the 

 controversy need not be followed into details. The truth 

 undoubtedly lies somewhere between these extreme views. 

 "Bacterial species exist, but they are all of the kind called 

 in the language of the science of classification 'Hi- 

 defined. 9 " 34 



The great body of facts instanced in the foregoing pages 

 furnish the answer to the question raised early in this sec- 

 tion, namely, does the inductive evidence warrant the sup- 

 position that homogeneous or structureless or organless 

 organisms actually exist? No, appears to be the only reply 

 permissible, and this in spite of the fact that the observable 

 organic world taken as a whole does undoubtedly show a 

 general simplification with diminution in size. 



If we feel compelled to speculate, but yet resolve, as we 

 must when fully committed to the inductive method, to hold 

 speculation to strict accountability to observational evi- 

 dence, then arc we driven to the conclusion that simplifica- 

 tion of structure and diminution in size of organisms go 

 hand in hand, but that neither ever reaches the vanishing 

 point, even though we know nothing about the limit of 



