572 Causes and Course of Organic Evolution 



even the penetrating and far reaching vision of an Indian or an 

 Arab has not started exceptional l^rain development in the 

 associated centers for these races, nor for the human race as a 

 whole. 



The auditory function, and its related brain center, though 

 often well-formed, by no means excels and often even is inferior 

 to that of many lower mammals. 



The high quality of the vocal cords, and the resulting capacity 

 for articulation, undoubtedly aid largely in conveying afferent 

 and efferent stimuli, as the efforts of every orator and vocalist 

 demonstrate. But we trust to prove that these organs of 

 Emerson's fancy have been greatly inferior, as primary agents, 

 to another source of stimulation. 



The skin as a chemotactic and mechanotactic recipient of 

 stimuli is highly important, but the stimuli thus received are 

 neither sufficiently varied nor elaborate as to constitute a 

 prime factor. 



Man then, like most animals, comes into intimate sense con- 

 tact with his outer world by six, possibly seven senses. These 

 are the smell and taste senses, or the chemotactic; the sight 

 sense or heliotactic (phototactic if preferred); the touch sense 

 or mechanotactic; the sound sense, or tonotactic; the pressure 

 sense or geotactic; and probably the thermotactic sense or heat- 

 perceptive. All of these except the tonotactic are common to 

 plants as well as animals. Further alike in plants and animals 

 some of the above senses, like the mechano-, the chemo-, and 

 the thermotactic are diffuse, that is, many areas or surfaces 

 over the body generally can act as receivers of such energy. 

 In both also sense-perception and response may be restricted 

 to limited areas, as the geotactic in root-tips of plants and in 

 one part of the auditory organ in animals; or the heliotactic 

 sense that is often highly diffuse in plants, but may be re- 

 stricted to o])tic centers in animals. 



Man then and the higher animals have evidently not at- 

 tained to their present high physical and mental organizations 

 by the addition of any special sense or senses of high quality 

 or importance, but rather by having their physical organiza- 



