Spatially Determined Reactions 227 



necessary to make a striped field just distinguishable 

 from a uniform field; (2) the just noticeable difference 

 between the width of stripes on two fields; (3) the just 

 noticeable difference in the direction of the stripes on two 

 fields; (4) the just noticeable difference in brightness 

 between two fields, one of which has stripes of equal bright- 

 ness, while the stripes on the other are of unequal bright- 

 ness. The chick's ability to distinguish a striped from a 

 plain field proved to be about one-fourth that of a monkey 

 or human being ; when the problem of distinguishing be- 

 tween striped fields whose stripes were of different widths 

 was presented, the monkey did ten times as well as the chick. 

 For differences in the direction of stripes, the threshold of 

 the chick was between twenty-five and thirty degrees ; the 

 monkey's was between two and five degrees : moreover, 

 the monkey learned the discrimination in twenty trials, 

 while the chick required 585. It seems practically certain 

 that the chick is not a fair representative of the bird family 

 as regards the clearness of its vision for form and size; 

 the eye of a hawk is a proverb for keenness, and the ability 

 of birds to find their food by vision demonstrates the high 

 development of their eyes in image-forming power. 



Among mammals, many dogs have been taught to dis- 

 tinguish printed letters on cards ; Sir John Lubbock's 

 poodle "Van" is a familiar example. Van learned to pick 

 out cards marked " Food, " "Bone, " " Out," "Water, " and 

 the like, and to present each on its appropriate occasion. 

 It took him ten days to begin to make the first step of dis- 

 tinguishing between a printed card and a plain one; in 

 a month this was perfected and in twelve more days, when 

 he wanted food or tea, he brought the right card one hundred 

 and eleven times and the wrong one twice. The second 

 mistake consisted in bringing the word "Door" instead of 



