308 The Animal Mind 



meant a. Thus Hans was trained to answer questions 

 other than those concerned with numbers. He showed 

 ability to do so with seeming intelligence, and to work 

 arithmetical problems. He was examined successively 

 by two commissions, and a psychologist on the second 

 commission, Pfungst, apparently solved the mystery of 

 Hans's behavior by showing that the person who put the 

 questions to the horse made unintentionally a slight move- 

 ment of the head when the proper number of taps had 

 been given, and that when such movements were inten- 

 tionally made, the horse responded to them. So the matter 

 rested, with the simple solution that the horse had, instead 

 of really thinking, merely reacted to involuntary signals. 

 After the death of Herr von Osten, Hans came into the 

 possession of Herr Krall, a business man of Elberfeld, who 

 was not satisfied with Pfungst's explanation, and besides 

 continuing the education of Hans, trained several more 

 horses, the most gifted of which were two Arabians named 

 Muhammed and Zarif. In two weeks Muhammed learned 

 to add and to subtract ; he passed in three days from multi- 

 plication and division to the use of fractions ; he acquired 

 remarkable skill in the extraction of square and cube roots, 

 and finally he as well as his fellow pupil began to offer 

 original observations. These performances occurred even 

 when the horses were prevented from seeing any one. Much 

 the same sort of phenomena are reported in the case of 

 Rolf, the Mannheim dog. The most recent reports of the 

 Elberfeld horses are less enthusiastic, and even claim fraud, 

 although not on the part of Herr Krall, whose disinterested- 

 ness seems accepted. It is impossible to determine just 

 what cues are responded to by these animals in their per- 

 formances, but aside from all the negative weight of the 

 evidence obtained under exact experimental conditions on 



