viii PREFACE. 
them would have been useless without some trustworthy 
principles of selection. The first and most obvious principle 
that occurred to me was to regard only those facts which 
stood upon the authority of observers well known as com- 
petent ; but I soon found that this- principle constituted 
much too close a mesh. Where one of my objects was to 
determine the upper limit of intelligence reached by this 
and that class, order, or species of animals, I usually found 
that the most remarkable instances of the display of intel- 
ligence were recorded by persons bearing names more or 
less unknown to fame. This, of course, is what we might 
antecedently expect, as it is obvious that the chances must 
always be greatly against the more intelligent individuals 
among animals happening to fall under the observation of 
the more intelligent individuals among men. Therefore I 
soon found that I had to choose between neglecting all the 
more important part of the evidence — and consequently in 
most cases feeling sure that I had fixed the upper limit 
of intelligence too low — or supplementing the principle of 
looking to authority alone with some other principles of 
selection, which, while embracing the enormous class of 
alleged facts recorded by unknown observers, might be 
felt to meet the requirements of a reasonably critical 
method. I therefore adopted the following principles as a 
filter to this class of facts. First, never to accept an alleged 
fact without the authority of some name. Second, in the 
case of the name being unknown, and the alleged fact of 
sufficient importance to be entertained, carefully to con- 
sider whether, from all the circumstances of the case as 
recorded, there was any considerable opportunity for mal- 
observation ; this principle generally demanded that the 
alleged fact, or action on the part of the animal, should be 
of a particularly marked and unmistakable kind, looking 
to the end which the action is said to have accomplished. 
