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THE AMERICAN NATURALIST [Vol. LVI 



in bacterial development, it is obvious that we can not 

 apply as justifiably as we do in the case of the higher 

 organisms the criteria of definitely directed evolution. 

 To be sure, we have a number of instances of the estab- 

 lishment of permanent new characters in bacteria, yeasts, 

 and other fungi through the influence of a change in en- 

 vironment. Most of these are, however, induced through 

 changes in the medium under artificial conditions and 

 they do not necessarily indicate a change in the direction 

 of improvement of the organism or of greater complexity 

 in its organization which may in turn point to the evolu- 

 tion of a higher form from a lower one. 



Despite the foregoing, it is probably well to examine 

 into certain facts with which we are familiar with regard 

 to microorganisms, and which may, perhaps, have a close 

 bearing upon what might be regarded as orthogenesis in 

 bacteria. The first fact to which I wish to refer is that 

 of parasitism. There can probably be very little doubt 

 that parasitism on the part of bacterial cells is not an 

 original, but an acquired character, using the term "ac- 

 quired" in its literal, and not technical, sense. If that is 

 granted, it would also follow that the acquirement of such 

 a characteristic by a microorganism would mean the 

 gradual adaptation of a bacterial cell from one kind of a 

 medium to another. It would mean the gradual acquire- 

 ment of partiality on the part of a microorganism 

 towards certain chemical substances, certain tempera- 

 tures, or certain other conditions which obtain only in a 

 living host and not in an inanimate medium. The steps, 

 gradual or rapid, by which the acquirement of such pecul- 

 iar characteristics on the part of the microorganism 

 would occur in its change from a saprophyte to a para- 

 site would almost seem to imply evolution in a dofiuite 

 direction. In a sense, therefore, we may reuar*] paia- 

 sitism in bacteria as an evidence of orthoufiiftic dcxclo])- 

 ment in such organism. It is, moreover, a case ot' evolu- 

 tion in a definite direction through the influences of en- 

 vironmental factors of the natural order and not those 



