112 THE AMERICAN NATURALIST [Vol. LVI 



dicated. It is not at all inconceivable that a green algal 

 cell may have adapted itself gradually to life within a 

 higher plant cell or within an animal cell, or to a sapro- 

 phytic existence in soil or other media devoid of light. 

 It may first have come there accidentally and then, 

 through the power to respond to such an environment 

 and to tolerate it, has gradually evolved new powers and 

 has lost some of its old powers. It is conceivable, there- 

 fore, that whether we regard parasites and saprophytes 

 among the bacteria as degraded forms or not, they 

 may be examples of evolution in a definite direction, pre- 

 sumably in this case in the direction of greater complex- 

 ity of function resulting from the urge of a constantly and 

 markedly changing and potent environment. 



Since the foregoing observations on orthogenesis in 

 bacteria have led me to enunciate in another form a theory 

 accounting for the origin of bacterial forms which has 

 been discussed before, I feel constrained to go one step 

 farther into that subject in order that my own views may 

 not be misunderstood. While the idea of accounting for 

 the origin of the bacterial cell from the single-cell alga 

 seems attractive and appears to be in consonance with cer- 

 tain well-known facts, there are several troublesome 

 features about it. In the first place, it assumes the de- 

 velopment of so complicated and intricate a substance as 

 chlorophyll before any form of living substance was 

 evolved. While this may, of course, have been the case, 

 it seems doubtful, in view of what we must consider to 

 be the highly specialized nature of the green pigment of 

 plants. In the second place, we have seen that the strong 

 argument in favor of the theory of the single-celled alga 

 as the primordial cell, or rather against the theory that 

 bacteria may have been such primordial cells, lies in the 

 well-known fact that most bacteria require organic com- 

 pounds as energy for their life processes and that no 

 organic matter could have been available without the 

 activity of chlorophyllous organisms. This argument, 

 however, overlooks two points, viz., first, the existence of 



