Nos. 622-623] ORGANIC EVOLUTION 



535 



The original development of the unicellular forms in 

 the northern hemisphere with their subsequent introduc- 

 tion to the southern hemisphere by aquatic birds, etc., is 

 well within the range of possibility and suggests that 

 even should forms with reverse rotations be entirely ab- 

 sent from south of the equator, the hypothesis which has 

 been proposed would by no means be invalidated. 



Having presented the general facts as to the behavior 

 of free-swimming microorganisms, it becomes advisable 

 to consider the explanations which may exist as to the 

 origin of the characteristic rotation. It seems impossible 

 to attempt to account for such a character on the ground 

 of "natural selection." One would be compelled to be- 

 lieve that the reverse rotation— the counterclockwise ro- 

 tation of positive northern forms— possessed an elimina- 

 tion value, an almost indefensible proposition, particu- 

 larly when forms like Stentor polymorphus and Phacus 

 longicauda are considered as well as forms in the south- 

 ern hemisphere which do not rotate in agreement with 

 theory. 



The most obvious explanation to be considered is that 

 based on the influence which the sun in its apparent daily 

 movement from east to west in the equatorial region may 

 be supposed to have exerted on the flagellum (Fig. 2). 

 This assumes that the flagellum is the orienting factor 

 and that the sun has induced in it an east- west rotary-like 

 or whip-like propelling movement. The consequent me- 

 chanical effect would be that in the northern hemisphere 

 forms with a positive light reaction would rotate clock- 

 wise and those with a negative light reaction would rotate 

 counterclockwise. Conditions would be reversed for 

 those which might be present in the southern hemisphere 

 during the evolutionary stage. 



Conversely, negatively pliototactic forms would de- 

 velop a reversed or counterclockwise rotation by means 

 of the influence of the light rays on the stroke of the light 

 avoiding flagellum and their modified organs, the cilia. 

 It is by no means necessary to believe that the stroke of 



