_Difficulty of Measuring Progress_.--In its larger generalization, progress may move in a straight line, but it has such a variety of expression and so many tributary causes that it is difficult to reduce it to any classification. Owing to the difficulties that attend an attempt to recite all of the details of human progress, philosophers and historians have approached the subject from various sides, each seeking to make, by means of higher generalizations, a clear course of reasoning through the labyrinth of materials. By adopting certain methods of marking off periods of existence and pointing out the landmarks of civilization, they have been able to estimate more truly the development of the race. Civilization cannot be readily measured by time; indeed, the time interval in history is of little value save to mark order and continuity. It has in itself no real significance; it is merely an arbitrary division whose importance is greatly exaggerated. But while civilization is a continuous quantity, and cannot be readily marked off into periods without destroying its movement, it is necessary to make the attempt, especially in the study of ancient or prehistoric society; for any method which groups and classifies facts in logical order is helpful to the study of human progress. _Progress May Be Measured by the Implements Used_.--A very common method, based largely upon the researches of archaeologists, is to divide human society into four great periods, or ages, marked by the progress of man in the use of implements. The first of these periods is called the Stone Age, and embraces the time when man used stone for all {36} purposes in the industrial arts so far as they had been developed. For convenience this period has been further divided into the age of ancient or unpolished implements and the age of modern or polished implements. The former includes the period when rude implements were chipped out of flint or other hard stone, without much idea of symmetry and beauty, and with no attempt to perfect or beautify them by smoothing and polishing their rough surface. In the second period man learned to fashion more perfectly the implements, and in some instances to polish them to a high degree. Although the divisions are very general and very imperfect, they map out the great prehistoric era of man; but they must be considered as irregular, on account of the fact that the Stone Era of man occurred at different times in different tribes. Thus the inhabitants of North America were in the Stone Age less than two centuries ago, while some of the inhabitants of the South Sea Islands are in the Stone Age during the present century. It is quite remarkable that the use of stone implements was universal to all tribes and nations at some period of their existence.