was shortened, until after seventy-five or one hundred trials, the animal immediately opened the door and seized the food.  In mechanical terms, the connection between “scrambling about the door” and “freedom to get the meat” became established by numerous repetitions until the originally disconnected elements were physiologically associated and made inseparable.  When animals like horses and seals and dogs are trained for the circus, it is by exactly the same method, for training consists merely in the establishment of a psychological sequence so that the performance of one series of acts leads mechanically to others.  Thus we learn that the psychological property called intelligence is the ability to establish wide relations between numerous activities which are themselves of a more or less complex nature; and we find also that because these elements are ultimately nerve-cell and sense-cell reflexes, an intelligent response is quite as machine-like as any and all of its elements.  A difference in degree of complexity and extent is the only thing that places intelligence apart from instinct and reflex action, for the units are the same in all cases,—­so far as science knows.

The apes are of the greatest value in providing the transition from the grade of intelligence to the human level where reason is found.  Whether or not a chimpanzee can reason at all is less important than the fact that its total “mental” powers are lower than those of man, and higher than those of inferior mammalia.  Apes are far more susceptible to training than cats and dogs, because their improved nervous mechanism enables them to establish a psychological sequence with greater facility.  If we are to judge by the facts at hand, these creatures possess a low order of mentality, like, but by no means equivalent to, that of man.

At the end of the comparative scale, we reach the human mind which is characterized by its ability to perceive and recognize far wider relations than those which are involved in intelligence.  Human consciousness is the stream of thoughts and feelings which constitute the immediate contents of mind.  In our own case, we know both the activities we perform and some of the internal phenomena with which such activities are connected.  Then we are impelled to compare the objective phenomena of action with the behavior of other men and of lower organisms, and if their behavior does not coincide with our own we are justified in believing that its direction lacks some of the elements we know about in our own case.  This is the method of comparative psychology, which establishes the conclusion that reason is the more complex term of a series to which reflex action, instinct, and intelligence directly lead.

Were we to study in detail the psychology of adult human beings, we would find only more truly that instinct and intelligence play a large part in our everyday mental life, and more certainly that even the highest reasoning powers we possess are only more complex in nature than the nervous processes of lower mammals and invertebrates.  Just as the nervous systems advance in physical or structural respects, so must their activities become more and more complex until the result is human faculty.