No. 643] 



ORTHOGENESIS IN BACTERIA 



107 



which are produced artificially. In respect, therefore, 

 of causal factors in the evolution of bacteria, we have 

 parasitism exemplifying the antithesis, so to speak, of 

 changes which we induce in bacteria in our artificial 

 media, or by changes in the environment. These observa- 

 tions would seem to possess cogency, not only in the case 

 of obligate parasitism, such as that characterizing the 

 organism of human tuberculosis, or of anthrax, or of the 

 fungus of wheat rust, but also that of what we may call 

 facultative parasitism in which the organism may have 

 adapted itself to life, both as a saprophyte and as a 

 parasite through the influence of certain chemical or physi- 

 cal-chemical agencies in its environment which have ren- 

 dered its protoplasm more highly adaptable than that of 

 the obligate parasite. We may, therefore, regard facul- 

 tative parasitism as an instance of orthogenetic evolu- 

 tion, just as we may so regard obligate parasitism. The 

 puzzling question which may, however, arise, from these 

 considerations is, which is the more advanced step in 

 orthogenesis in parasitic bacteria. Is the obligate para- 

 site the more advanced form, or is the facultative ])arasite 

 the more advanced form? While many would ])r()l)ably, 

 on first impulse, regard the former as the correct answer, 

 it does not necessarily follow that such is the case. Cer- 

 tainly in this regard, a uicat luaiiy more facts are needed 

 before any definite stalcnu iits can l)e made. 



Examples of other casus of orthogenetic evolution in 

 bacteria other than tlie ease of parasitism, which I have 

 just discussed, may be nmltiplied ad Uhitum. But, owing 

 to limitations of time and space, it will suffice to mention 

 a few only. 



The adaptation of bacteria to the physiological char- 

 acteristic of nitrogen fixation, such as is possessed by all 

 the Azotohncter species, and the CJo.^fnd'mm species and. 



have been the result of aiiythi.i- else' than a cas.. of .h'f- 

 initely directed evolution. This was j.rohnhly accom- 

 plished through the influence <^f an en\ ironmi iit in which 



