224 The Animal Mind 



the one of larger area ; if negatively phototropic, away from 

 it (132). 



Turtles showed a remarkable keenness of discrimination 

 in the study made by Casteel (119), in which they were 

 offered the choice of two compartments faced with card- 

 boards carrying black lines on a white ground. Two turtles 

 learned to discriminate between vertical lines eight milli- 

 meters in width and vertical lines two millimeters in width, 

 and one gifted animal learned to distinguish, first, lines 

 eight millimeters wide from lines one millimeter wide, then 

 between a width of four millimeters and a width of one 

 millimeter, then between four and two, and finally between 

 three and two millimeters. Chicks proved equal to a 

 discrimination between a standard circle six centimeters in 

 diameter and one from one-fourth to one-sixth larger. The 

 relative brightness of the circles was varied so that the chicks 

 could not use this as a basis for their choices (102). White 

 rats can discriminate circles thirty millimeters in diameter 

 from circles fifty millimeters in diameter, and squares 

 twelve centimeters a side from squares one centimeter a 

 side (411). Discrimination of boxes differing in size but 

 alike in form, placed in a row along a board, food having 

 been put in one, was imperfectly learned by two Macacus 

 monkeys ; the errors leaned in the direction of taking the 

 larger vessel (401). Raccoons were taught to distinguish 

 perfectly between two cards, one 6| X 6| inches square 

 and the other 4^ X 4^ , shown successively. The animals 

 were to climb on a box for food when the larger card was 

 shown and to stay down when the smaller one appeared. 

 As we shall see later, L. W. Cole, the experimenter, thinks 

 the learning gave evidence not only of a spatial image, but 

 of a memory image (134). 



One apparent effect of size upon visual perception relates 



