To the intelligent man with an interest in human nature it must often appear strange that so much of the energy of the scientific world has been spent on the study of the body and so little on the study of the mind. 'The greatest thing in man is mind,' he might say, 'yet the least studied.' Especially remarkable seems the rarity of efforts to trace the evolution of the human intellect from that of the lower animals. All the animals like fishes, reptiles, birds and various mammals, notably dogs, cats, mice and monkeys are manifest fundamentally the same sort of intellectual life. From the lowest animals of which we can affirm intelligence up to man this type of intellect is found. The growth in the number, speed of formation, permanence, delicacy and complexity of associations possible for an animal reaches its acme in the case of man.