176, Animal Intelligence 
The subjects of the experiments were three South Ameri- 
can monkeys of the genus Cebus. At the time of beginning 
the experiment No. 1 was about half grown, No. 2 was about 
one fourth full size and No. 3 was about half grown. No.1 
was under observation from November, 1899, to February, 
1906; No. 2 and No. 3 from October, 1900, to February, 
1go1. No. 1 was during the period of experimentation 
decidedly tame, showing no fear whatever of my presence 
and little fear at being handled. He would handle and 
climb over me with no hesitation. No. 2 was timid, did 
not allow handling, but showed no fear of my presence and 
no phenomena that would differentiate his behavior in 
the experiments discussed from that of No. 1, save much 
greater caution in all respects. No. 3 also showed no fear 
at my presence. Any special individual traits that are of 
importance in connection with any of the observations will 
be mentioned in their proper places. No. 1 was kept until 
June, 1900, in my study in a cage 3 by 6 by 6 feet, and was 
left in the country till October, 1900. From October, 1g00, 
all three were kept in a room 8 by 9 feet, in cages 6 feet tall 
by 3 long by 2.6 wide for Nos. 1 and 2, 3 feet by 3 feet by 20 
inches for No. 3. I studied their behavior in learning to. 
get into boxes, the doors to which could be opened by 
operating some mechanical contrivance, in learning to 
obtain food by other simple acts, in learning to discriminate 
between two signals, that is, to respond to each by a dif- 
ferent act, and in their general life. 
Following the order of the ‘Animal Intelligence,’ I shall 
first recount the observations of the way the monkeys 
learned, solely by their own unaided efforts, to operate 
simple me mechanical contrivances. 
Besides a number of boxes such as were used with the 
dogs and cats (see illustration on p. 30), I tried a variety 
