108 THE AMERICAN NATURALIST [Vol. LVI 



it was impossible for the organisms existing therein to 

 live without acquiring a power of employing energy exist- 

 ent in carbonaceous material to fix atmospheric nitrogen 

 and make it available for their own life processes. The 

 next case which may be cited is that of the lactic-acid 

 bacteria, which possess the power of transforming lactose 

 (or milk sugar) into lactic acid. These cells are not in 

 form or otherwise in function appreciably different from 

 any other bacteria with which we are acquainted. They 

 have, nevertheless, this specific and peculiar power to 

 which I have alluded. Is it likely that they have acquired 

 this power through any other influence than the influence 

 of environment which operated in a definite direction and 

 hence orthogenetically ? The sulphur bacteria, or par- 

 ticularly those species of sulphur bacteria which have the 

 power of oxidizing sulphur to sulphuric acid, are another 

 case in point. The nitrifying bacteria are still another 

 case in point. The iron oxidizing bacteria are still an- 

 other case, and so we might go on and mention very many 

 classes of bacteria, in each case of which there is a def- 

 inite, distinct, and strikingly peculiar functional power 

 which could not well have been developed without the 

 influence of some environmental factor or factors opera- 

 ting in a definite direction. It is not so easy on the mor- 

 phological side to give examples like those which I have 

 just cited from the point of view of function of the bac- 

 teria. The reason for that has already been touched on 

 above, namely, the simplicity of form and particularly 

 the slight variety in form which characterizes the bac- 

 teria. In fact, it is my conviction that it is best to ignore, 

 largely, morphological factors in bacteria when we study 

 the problem of bacterial evolution. My conviction arises 

 from a study of many and varied experiments which I 

 can not discuss here. 



Viewing our subject, then, from the standpoint that 

 orthogenesis in bacteria would be concerned with pro- 

 gressive changes in the organism, principally physio- 

 logical, due to its response to changes in environment, it 



