It remains to add a few words on the principles which I have laid down for my own guidance in the selection and arrangement of facts. Considering it desirable to cast as wide a net as possible, I have fished the seas of popular literature as well as the rivers of scientific writing. The endless multitude of alleged facts which I have thus been obliged to read, I have found, as may well be imagined, excessively tedious; and as they are for the most part recorded by wholly unknown observers, the labour of reading 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 competent; 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 intelligence 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 consider 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. Third, to tabulate all important observations recorded by unknown observers, with the view of ascertaining whether they have ever been corroborated by similar or analogous observations made by other and independent observers. This principle I have found to be of great use in guiding my selection of instances, for where statements of fact which present nothing intrinsically improbable are found to be unconsciously confirmed by different observers, they have as good a right to be deemed trustworthy