122 Causes and Course of Organic Evolution 



regard Euglena as a free-swimming plant, as a transition type 

 from such to the animal side, or as a flagellate infusor, the 

 presence in it, as in many other motile alga.% of a red "eye-spot" 

 distinct from the chromatophore, is proof that the chloro- 

 phylloid areas are not the only heliotactic centers. Engel- 

 mann's experiments (38) with this organism make it "clear 

 that the anterior end is more sensitive to light than the re- 

 mainder of the body." 



But that even plant parts which are colorless are highly 

 heliotactic is proved by the aerial growths of Pilobolus, Mucor, 

 etc., though as yet we are unaware whether definite helio- 

 plasts are the cause of the movement. According to the experi- 

 ments of Jennings {Jf2: 133), most infusors that were studied 

 seem to be heliophobic, or may possibly be heliotactic to feeble 

 light. He says: "Every time that the anterior end swings into 

 the light, the avoiding reaction is caused; the animal therefore 

 swings its anterior end in a large circle, trying many directions. 

 Every time it swings its anterior end away from the source 

 of light into the shadow of its body, on the other hand, no 

 reaction is produced; the position thus reached is retained. 

 This process continues, the animal trying new directions every 

 time its anterior end swings toward the light, until in a short 

 time the anterior end must inevitably become directed away 

 from the light. In this position the anterior end is no longer 

 subjected to changes in illumination, for the axis of the course 

 coincides with the axis of the light rays, and the body maintains 

 a constant angle with the axis of the course. The amount of 

 light received by the anterior end therefore remains constant. 

 Hence there is no further cause for reaction, and the organism 

 retains the position with anterior end directed away from the 

 source of light." 



W hile in higher algse and still more evolved groups the helio- 

 tactic sense becomes as widespread or diffuse as are the chlor- 

 oplast centers, in the main lines of evolving animals it becomes 

 increasingly restricted to one, two, or more anterior heliotactic 

 organs, the eyes. So, in the Rotifera, the Turbellaria, the 

 Nemertinea, and upward through the main vertebrate line 



