212 Causes and Course of Organic Evolution 



phototactic perception resides wholly in the "eye-spot" that 

 is located there. The molecular changes then, established 

 there, are the first indications of a definite role of events that 

 will culminate in motion of the animal in a definite direction. 



Amongst the multicellular Caryota, not only is proenviron- 

 ment exhi])ited on every hand; in many cases each proen- 

 vironal response is a resultant of two, three, or more com- 

 pounded stimuli. 0\nng also to size of the parts and ease of 

 observation the results are already of great value, and are 

 recorded in text-books on plant and animal physiology. Fur- 

 ther, as has been pointed out already in treating of the "stock- 

 in-trade" of plants and animals (p. 164), the chief proenvironal 

 factors can be traced to act throughout the entire scale of 

 higher organic life, and most of them are known as the tropic 

 or tactic stimuli. Some of these, however, are often treated 

 of in text-books as simple stimuli, when in reality constituent 

 rays of lumic energy may cause very diverse results. Thus 

 we often speak of natural or white or mixed light as causing 

 either positive or negative response phenomena, when strictly 

 the red or red-orange constituent may start one response, the 

 green another, and the blue- violet a tliird; or these may all 

 cooperate in producing one satisfied result up to a certain 

 degree of intensity, and thereafter the blue-violet rays may 

 become liighly irritant and hurtful, while the red-orange may 

 continue to exercise a beneficial effect. 



Short reference has already been made to the fungus Pilo- 

 hohis. As this develops its spore bearing stalk, if the manurial 

 mass on wliich it is gro"v\'n be illuminated equally from above, 

 the fungus will proenviron a course that is directly upward, 

 since the stalk is apogeo tropic or is negatively responsive to 

 the earth's mass, while it is also heliotropic to light, and thus 

 proenvirons a course that is in line with the light rays and 

 gravic stimulus. But, if the well-known experiment of illum- 

 inating a crop of it through a minute orifice on one side of an 

 otherwise darkened chamber be arranged, then the heliotropic 

 lesponse is so great that the apogeotropic one is canceled or 

 modified to an angular degree that accurately brings the axis 



