643] ORTHOGENESIS IN BACTERIA 



111 



exceptional forms of bacteria, some of which I have 

 named above, the physiological characteristic is either 

 that of a saprophyte or of a parasite. It seems obvious 

 to me that neither a saprophytic nor a parasitic organism 

 can well be expected to originate in an environment 

 which is devoid of elaborated organic matter. Subject 

 to considerations which I shall discuss later, we must, 

 therefore, accept one or two conclusions with regard to 

 the origin of bacteria in the scale of evolution of organ- 

 isms generally. Either they are the most primitive 

 forms of organisms which have lost their primitive 

 powers of living in purely inorganic media, or they are 

 a much more advanced form of life which came to be after 

 other organic forms had for some time been developing 

 on the earth's surface. The first possibility is merely 

 tantamount to saying that some cells of the most primi- 

 tive forms have gradually adapted themselves to either a 

 saprophytic or a parasitic existence and, therefore, is of 

 little assistance to us. The correctness of the second con- 

 clusion, however, would seem to depend on many little- 

 known factors. Still, it is the belief of many scholars. 

 Putting the matter in another way for greater clarity 

 and emphasis, I may state it as the opinion of several 

 plant physiologists who have speculated upon this sub- 

 ject, that the primitive forms of living cells were prob- 

 ably those which could live in a purely inorganic medium. 

 Obviously, such cells must have been limited to the group 

 which we now call the autotrophic organisms, and of the 

 autotrophic organisms, since the higher plants are cer- 

 tainly a very advanced form, we must have had some- 

 thing very much sim])ler. and tlie natural conclusion is 

 that such a simpler form of (u^uaiiism iiinst have been 

 the single-celled green al.ua. oi- form- closely similar to 

 it. If we assume that >uch was tho vn<i\ then it is not 

 difficult to ])ro])ose a st-lu'im" of fvolutioii ot' the bacteria 

 which involves the .m-adual cliaimv of tlie unicellular 

 g-reen alga^ into a variety cf hartci'ial foi'iiis through the 



